Dynamic Six-Degrees-Of-Freedom Intervertebral Spinal Disc Prosthesis

ABSTRACT

The subject invention provides a modular six-degrees-of-freedom spatial mechanism for spinal disc prosthesis, with up to three rotational and up to three translational degrees-of-freedom within the entire workspace of a Functional Spinal Unit (FSU). The prosthetic disc mechanism consists of up to three independent cylindrical joints, each joint providing one linear and one rotational degree of freedom. The superior and inferior vertebral plates of the device anchor to the superior and inferior vertebrae of an FSU and the device maintains an inseparable mechanical linkage between those vertebrae for all normal motions and positions of the FSU. The device utilizes resilient spring elements, components that self-adjust in position and orientation, in conjunction with a fiber reinforced boot and toroidal belt, as well as a unique hydraulic damping system to accommodate dynamic and static forces and sudden shocks on the FSU. The device can adjust to maintain the appropriate, but changing, intervertebral spacing during normal FSU motion. Scaling, conjoined with cushioned, joint-limit stops, allows the device to realize almost any nominal spinal articulation, from the cervical to lumbar regions.

BACKGROUND OF INVENTION

Spinal disc herniation, a common ailment, often induces pain, as well asneurologically and physiologically debilitating processes for whichrelief becomes paramount. If conservative treatments fail, the moredrastic measures of discectomies and spinal fusion may be indicated. Thelatter treatment, while providing short term relief, often leads toexcessive forces on facet joints adjacent to the fusion and createsfurther problems over time. Drastic treatments are usually unable torestore normal disc function. The loss of disc function has led to anumber of disc prostheses that attempt to provide natural motion.

The literature documents that the Instantaneous Axis of Rotation (IAR)during sagittal rotation of the superior vertebra with respect to theinferior vertebra of a Functional Spinal Unit (FSU) in the cervicalspine moves significant distances during flexion and extension of thespine (Mameren H. van, Sanches H., Beursgens J., Drukker, J., “CervicalSpine Motion in the Sagittal Plane II: Position of Segmental AveragedInstantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992,Vol. 17, No. 5, pp. 467-474). This motion varies widely betweenfunctional spinal units on an individual spine and between individualsand depends on age, time-of-day, and the general health and condition ofthe intervertebral discs, facet joints and other components of the FSUand spine. A moving IAR means that the superior vertebra both rotatesand translates while moving with respect to the inferior vertebra of anFSU. Natural spinal motions place severe requirements on the design of aprosthetic disc; simple rotational joints are not able meet thoserequirements.

In addition, motion coupling between axial and lateral bending and otherfunctional spinal units involved in the overall spinal motion increasesthe complexity and difficulty in developing a prosthetic discreplacement that realizes natural spinal motion. The complex facetsurfaces in an FSU significantly influence and constrain sagittal,lateral and axial motions. The orientation of these facet surfaces varywith FSU location in the spine and induce wide variations in motionparameters and constraints. The complex motion of a superior vertebrawith respect to the associated inferior vertebra of an FSU, certainly inthe cervical spine, cannot be realized by a simple rotation or simpletranslation, or even a combination of rotation and translation along afixed axis, and still maintain the integrity and stability of the FSUand facet joints.

One advantage of a general motion spatial mechanism of a discprosthesis, as described in this application, is that it solves thenatural motion problem for disc prostheses and offers a scalablemechanism for disc replacement without loss of general motioncapabilities in the FSU.

Researchers have attempted to design a successful intervertebral discfor years. Salib et al., U.S. Pat. No. 5,258,031; Marnay, U.S. Pat. No.5,314,477; Boyd et al., U.S. Pat. No. 5,425,773; Yuan et al., U.S. Pat.No. 5,676,701; and Larsen et al., U.S. Pat. No. 5,782,832 all useball-and-socket arrangements fixed to the superior and inferior platesrigidly attached to the vertebrae of an FSU. However, these designslimit motion to rotation only about the socket when the two plates arein contact. As the literature points out (Bogduk N. and Mercer S.,“Biomechanics of the cervical spine. I: Normal kinematics”, ClinicalBiomechanics, Elsevier, 15 (2000) 633-648; and Mameren H. van, SanchesH., Beursgens J., Drukker, J., “Cervical Spine Motion in the SagittalPlane II: Position of Segmental Averaged Instantaneous Centers ofRotation-A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp.467-474), this restricted motion does not correspond to the naturalmotion of the vertebrae, for either sagittal plane motion, or forcombined sagittal, lateral and axial motion. Further, when the twoplates, as described in the cited patents, are not in contact, thedevices are unable to provide stability to the intervertebral interface,which can allow free motion and lead to disc related spondylolisthesis,FSU instability, and excessive facet loading.

As a further elaboration on the many ball-and-socket configurations,consider Salib et. al. (U.S. Pat. No. 5,258,031) as an example ofprevious efforts to address this problem. The Salib ball-and-socketarrangement only provides 3 independent axes of rotation and notranslation when engaged.

During complex motions of an FSU, the superior vertebra, in general,requires translation along three independent directions. A sliding ovatestructure in an oversized socket cannot perform such general translationmotions, either, as it must engage in a trajectory dictated by itssocket's geometrical surface and does not change the deleterious effectsthat may occur on the facet joints of the unit.

Currently known devices appear to have similar motion and instabilitylimitations, such as the rocker arm device disclosed by Cauthen (U.S.Pat. Nos. 6,019,792; 6,179,874; 7,270,681), the freely moving slidingdisc cores found in the Bryan et al. patents (U.S. Pat. Nos. 5,674,296;5,865,846; 6,001,130; and 6,156,067) and the SB Charité™ prosthesis, asdescribed by Buttner-Jantz K., Hochschuler S. H., McAfee P. C. (Eds),The Artificial Disc, TSBN 3-540-41779-6 Springer-Verlag, BerlinHeidelberg New York, 2003; and U.S. Pat. No. 5,401,269; andBuettner-Jantz et al. U.S. Pat. No. 4,759,766). In addition, the slidingdisc core devices of the Bryan et al. and SB Charité™ devices do notpermit natural motion of the joint for any fixed shape of the core.

With the above-described prosthetic devices, when the FSU extends, theprosthesis's sliding core, in some cases, generates unnaturalconstraining forces on the FSU by restricting closure of the posteriorintervertebral gap in the FSU. Further, the core does not mechanicallylink the upper and lower plates of the prosthesis and is unable tomaintain the intervertebral gap throughout the range of motion. Suchconditions can contribute to prosthetic disc spondylolisthesis. Ingeneral, unconstrained or over-constrained relative motion between thetwo vertebral plates in a prosthetic disc can contribute to FSUinstability over time.

Static loading in current prosthetic disc technology appears to beminimal and limited to mostly rigid support. For example, load bearingand shock absorption in the SB Charité™ design and others (e.g. Bryan etal., U.S. Pat. No. 5,865,846) rely on the mechanical properties of theresilient, ultra-high-molecular-weight polyethylene core to provide bothstrength and static and dynamic loading. The rigidity of the slidingcore appears to offer little energy absorption and flexibility to meetthe intervertebral gap requirements during motion, and may likelygenerate excessive reaction forces on the spine during flexion, forcesthat can potentially produce extra stress on facet joints and effectmobility.

More recent attempts to provide dynamic and static loading capability istaught in the series of patents by Ralph et al (U.S. Pat. Nos.6,645,249, 6,863,688, 6,863,688, 7,014,658, 7,048,763, 7,122,055,7,208,014, 7,261,739, 7,270,680, 7,314,487) wherein the force restoringmechanism begins with a multi-pronged domed spring between two platesand ends with a wave-washer as the force restoring element. Themulti-pronged domed spring employs a ball-and-socket arrangement on theupper plate and allows relative rotations between the spring-lower plateand the upper plate. This arrangement, during normal FSU operation,places moments of force on the spring that tend to distort the springand place high stresses on the set screws holding the spring down. Theeffects of force moments on the prongs and the dome spring is mitigatedby later designs where various modifications of the spring element, asfor example the spiral Belleville washer in U.S. Pat. No. 7,270,680,provides the spring more resilience to moments of force. As taught inthese patents, the motion of the upper plate is limited to compressionand rotation. Lateral and sagittal translations are not accommodated andso general motion in the FSU is not enabled by the device.

The work of Errico et al (U.S. Pat. Nos. 6,989,032, 7,022,139,7,044,969, 7,163,559, 7,186,268, 7,223,290, and 7,258,699) elaborates onthe mechanical design of the patents of Ralph et al. A speciallydesigned Belleville type washer provides a restoring force tocompressions. Rotations of the superior plate of the device in a fixedball-and-socket arrangement transfers moments of force about the washercentral axis to a rigid structure. It is notable that the instruction inthese designs specifically proscribes lateral motions (sagittal andlateral translation). Errico et al. employ a tapered projection attachedto the ball to limit rotation angles.

Another approach to incorporate dynamic and static force response istaught by Gauchet (U.S. Pat. Nos. 6,395,032, 6,527,804, 6,579,320,6,582,466, 6,582,468, and 6,733,532) wherein a hydraulic system providesshock absorption by means of a cushion between two plates containedwithin sealed flexible titanium bellows. Gauchet suggests the bellowscan be designed to accommodate lateral forces and axial rotation that ispermitted by the cushion, which, to allow sliding motion, is notattached to at least one plate. The titanium bellows can accommodatesome axial rotations, but do not seem suitable for other rotations,which can cause excessive stresses on the bellows. A cushion internal tothe cylinder, being flexible and not attached to at least one plate, canaccommodate any rotation (U.S. Pat. Nos. 6,582,466 and 6,733,532).

Fleishman et al in U.S. Pat. Nos. 6,375,682 and 6,981,989 utilizehydraulic action coupled with a flexible bellows to mitigate suddenforces. The bellows concept is similar to that of Gauchet.

Eberlein et al (U.S. Pat. No. 6,626,943) utilizes a fiber ring toenclose a flexible element. The forces and moments of force are absorbedby the ring and the flexible element. The device taught in thisinvention uses a boot in much the same manner as Eberlein's fiber ring.Other inventions teach this concept as well, namely, Casutt in U.S. Pat.No. 6,645,248. Diaz et al (U.S. Pat. No. 7,195,644) also uses a membraneand enclosed cushioning material in their ball and dual socket jointdesign.

Middleton suggests a variety of machined springs as the centralcomponent of a disc prosthesis in U.S. Pat. Nos. 6,136,031, 6,296,664,6,315,797, and 6,656,224. The spring is notched to allow static anddynamic response primarily in the axial direction of the spring. But,lateral and sagittal translations and general rotations appear to beproblematic in these designs. The ability of such springs to tolerateoff-axis compression forces may also be problematic.

Gordon instructs deforming a machined spring as the principle separatingand force management component (U.S. Pat. Nos. 6,579,321, 6,964,686, and7,331,994). In U.S. Pat. No. 7,316,714, also to Gordon, the emphasis ison posterior insertion of a disc prosthesis that can provide appropriatemotion. However, this latter design does not appear to accommodate forstatic and dynamic loading and there appears to be no accommodation forlateral and sagittal translations.

Zubok instructs in U.S. Pat. No. 6,972,038(Column 3; Line 35) that “ . .. the present invention contemplates that with regard to the cervicalanatomy, a device that maintains a center of rotation, moving orotherwise, within the disc space is inappropriate and fails to properlysupport healthy motion.” This may be true as long as translations withinthe prosthesis mechanism do not adequately compensate for the totalmotion induced by an LAR outside of the disc space. Several approachesby Ferree (U.S. Pat. Nos. 6,419,704, 6,706,068, 6,875,235, 7,048,764,7,060,100, 7,201,774, 7,201,776, 7,235,102, 7,267,688, 7,291,171, and7,338,525) primarily instruct how to cushion a prosthetic FSU in variousways. An exception is U.S. Pat. No. 6,706,068, which describes a designto perform certain kinematic motion of a disc prosthesis without dynamicor static cushioning support, and U.S. Pat. No. 7,338,525, whichinstructs on anchoring a disc prosthesis.

Aebi incorporates what essentially amounts to a hook joint (orthogonalrevolute joints) in EP1572038B1 as the means for realizing motion. Whilethe Aebi arrangement of revolute joints does allow for sagittal andlateral rotations, it does not engage in the remaining four degrees offreedom in three-space, namely, sagittal, lateral, and axialtranslations along with axial rotations.

Mitchell (U.S. Pat. No. 7,273,496B2) uses two revolute joints by meansof orthogonal cylinders placed on top of each other and embedded as acrossbar element between vertebral plates with cavities for acceptingthe crossbar. This device has the limitations of motion similar to theAebi device, and the further limitation of not linking the two platestogether with the crossbar.

Khandkar (U.S. Pat. No. 6,994,727 B2) provides two orthogonal convexcurvate bearing structures, with offset cylindrical radii of curvature,placed between the vertebral plates. An insert, with orthogonal,variable-curvature concave bearing surfaces, is placed between, andgenerally conforms to, the orthogonal convex bearings on the vertebralplates. This arrangement of bearings allows sagittal, lateral, and axialrotations of various ranges, dictated by the curvate bearing structuresand the insert. The variable curvate surfaces allows some lateral andsagittal translations with FSU distractions, utilizing normal spinalforces to resist the distraction and, hence, the motion. There is nocontrol on the forces involved, so this method could lead to possiblestress on other spinal joints. The inserted device is not kinematicallychained to the rest of the device and can possibly be spit out.Although, as instructed, the device is self-correcting within a limitedrange, tending towards a stable equilibrium established for the devicein normal position. The variable curvatures can result, typically, inline-contact bearing manifolds that will wear the surfaces, possiblycausing changes in the performance and characteristic motion of thedevice.

DiNello (US Publication No. 2006/0136062A1) instructs on how to adjustheight and angulation of a motion disc after implantation.

With respect to the lower vertebra in an FSU, all possible, natural lociof motion of any four non-planar, non-collinear points located in thesuperior vertebra define the natural workspace of a FSU. This workspacevaries from one FSU to another on the spine, creating considerablespinal disc prosthesis design problems.

The FSU workspace boundary is dictated by the sagittal, lateral andaxial angle limits reported in the literature (Mow V. C. and Hayes W.C., Basic Orthopaedic Biomechanics, Lippincott-Raven Pub., N.Y., 2^(nd)Addition, 1997). However, these angle limits do not reveal theunderlying complex motion between two vertebrae in an FSU. The study byMameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical SpineMotion in the Sagittal Plane II: Position of Segmental AveragedInstantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992,Vol. 17, No. 5, pp. 467-474 demonstrates this complexity in the cervicalspine, even when the motion is restricted to flexion and extension.

In light of the above observations and limitations, it can beappreciated that there is a need for a spinal disc prosthesis that canaccommodate a broader range of motions, while maintaining disc stabilityand integrity under static and dynamic loads.

BRIEF SUMMARY

The subject invention provides a spinal disc prosthesis capable ofproviding spatial movement with up to 6 degrees of freedom (FIG. 1 andFIG. 2). In a preferred embodiment, the device of the subject inventionfacilitates sagittal, lateral, and axial vertebral displacements androtations when utilized in the spine of a patient.

In one embodiment, the modular spinal disc prosthesis of the subjectinvention comprises superior and inferior vertebral plates, as well as aflexible, boot-protected, replaceable 6-DOF modular prosthetic discmechanism (linkage). The devices of the subject invention can achieve upto 6 degrees of freedom, including up to 3 independent rotationaldegrees of freedom and up to 3 independent linear degrees of freedom,such that the device of the subject invention facilitates sagittal,lateral, and axial vertebral displacements and rotations when utilizedin the spine of an animal. The modular prosthetic disc mechanism of thesubject invention can comprise three orthogonal cylindrical jointelements for general positioning and orienting of the superior vertebrawith respect to the inferior vertebra of a Functional Spinal Unit.

In one embodiment, the cylindrical joints kinematically connect asuperior and inferior vertebral plate by means of mechanicallyinterlocking and inseparable cylindrical joint elements arrangedmutually orthogonal to each other. Thus, the elements remain attached toone another and the vertebral plates throughout natural FSU motion. In afurther embodiment, the vertebral plates can be rigidly fixed to thesuperior and inferior vertebrae of a Functional Spinal Unit (FSU) or,with obvious modification of the device's vertebral plates, modularlyfixed to such plates, as discussed in Doty (U.S. Pat. No. 7,361,192),which is hereby incorporated by reference. In a still furtherembodiment, displacements along the axial axis, a line perpendicular tothe axial plane of the FSU (not the patient body axial axis), arise fromcompressing a spring-dashpot element that also constitutes a centralaxial cylindrical joint whose components constitute a central shockabsorbing system. Hydraulic portals within the device can alsofacilitate shock absorbing characteristics while at the same timeforcing a bio-lubricant, or other substance, to flow through and aroundthe components of the device. This central axial cylindrical joint,which includes a combined dual cylinder and a spring stack, provide acolumn element that resists shear forces and promotes the rotation andtranslation of the various joint elements when the FSU is subjected toshear forces.

To further assist with shock-absorption, a flexible, elastomer boot canbe utilized to surround the functional elements of the prostheticdevice. The boot can further be sealed such that surrounding bodilyfluids cannot contact the functional elements of the prosthetic device.In still a further embodiment, the sealed boot can contain fluids orother substances to lubricate the functional elements of the prostheticdevice. The central cylindrical joint, can further act as a hydraulicpump, to helps divert compression shocks to the walls of the boot,causing the boot to bulge and absorb some of the energy of the shock.

To further assist the boot and central cylindrical joint in resistingshocks and arbitrary FSU force loads, an internal toroidal-beltcushioning element can be utilized with the subject invention.

Thus, the present invention provides an articulated, modular6-Degrees-of-Freedom (6-DOF) spatial mechanism for intervertebral spinaldisc prosthesis that provides highly advantageous spatial motion betweenupper and lower vertebrae of an FSU with static and dynamic loadcapabilities.

The device of the subject invention can be used to assist in maintainingnatural spinal flexibility and motion during simultaneous, dynamicallychanging, curvilinear axial, lateral and sagittal rotations andtranslations, regardless of the details and wide variations of thatmotion in a patient.

The unit can also assist in accommodating variable disc spacing understatic and dynamic load during normal FSU operation. For example, thedisc spacing under static load in the normal spinal position can beselected by adjusting certain components of the device. The inventioncan absorb compression shocks, sustain static loads, respond to dynamicloads, help alleviate spinal cord and nerve root compression, resisttorsion and extension forces and reduce excessive facet joint stress andwear.

The mechanism's components, when coupled together, form a device thatpreserves its own mechanical integrity, connectedness (inseparablekinematic chain), and motion properties throughout the biologicallyconstrained motion space (workspace) of the FSU. The complete generalityof the device allows for modifying the range of the mechanism's motionparameters and workspace, physical size, material composition, andmechanical strength to suit ordinary mechanical applications as well asspinal disc prosthetics.

The complete 6-DOF motion capability of the prosthetic disc linkagemechanism is able to allow natural motions dictated by the muscles andligaments of the spine. Throughout normal motion, the system of thesubject invention stabilizes the FSU because of its ability to maintaincontinuity of mechanical connection between the superior and inferiorvertebrae while at the same time providing load bearing and permittingmotion only within the nominal disc operating range or workspace. Themechanical continuity is realized by a kinematic chain of inseparablejointed elements.

BRIEF DESCRIPTION OF DRAWINGS

In order that a more precise understanding of the above recitedinvention be obtained, a more particular description of the inventionbriefly described above will be rendered by reference to specificembodiments thereof that are illustrated in the appended drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered as limiting in scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 depicts one embodiment of an assembled disc prosthesis of thesubject invention in perspective. Visible elements of this embodiment,shown in this view, are a superior vertebral plate 12-1, afiber-reinforced, resilient boot 8-1 and an inferior vertebral plate1-1. Also shown, are a locking key 13-1 that can be pressure fit, weldedor otherwise fixedly positioned into 1-1 and projects interiorly into asagittal rotating cylinder 2-1 (not shown here), which rotates about thecurvate projection of the locking key.

FIG. 2 shows an embodiment of the invention without the boot, revealingan inferior hydraulic cylinder wall 4-1 and a superior hydrauliccylinder wall 6-1.

FIG. 3 illustrates an embodiment of a boot of the subject invention ofresilient, fiber-reinforced elastomer matrix that can be firmly attachedto the superior and inferior vertebral plates (12-1 and 1-1).

FIG. 4 shows an embodiment of clamping and reinforcing rings embeddedinto the boot matrix. Various fiber weaving schemes can use the rings asa starting platform for the weave.

FIG. 5 illustrates a spherical cross weave 8-1-3 embodiment for a boot.

FIG. 6 shows an embodiment of the invention, without the boot, in fullflexion, that is, maximum rotation of sagittal rotation cylinder 2-1(see FIG. 9) about the sagittal axis. Note, in this embodiment, thetranslation and rotation of the superior vertebral plate with respect tothe inferior vertebral plate and the collapse of the superior hydrauliccylinder wall 6-1 over the inferior hydraulic cylinder wall 4-1 in atelescoping manner. Also, observe that, in this position, a portion ofthe spring platform and hydraulic cylinder base 3-1 projects slightlyabove the inferior vertebral plate 1-1.

FIG. 7 portrays an embodiment of the invention, without the boot, innormal position wherein the superior 12-1 and inferior 1-1 vertebralplate parallel each other and the superior hydraulic cylinder wall 6-1slides up to reveal some of the inferior hydraulic cylinder wall 4-1.

FIG. 8 illustrates an embodiment of the invention in full extension withthe superior hydraulic cylinder wall 6-1 at its maximum extension withrespect to the inferior hydraulic cylinder wall 4-1. The two cylindershave locking rings or interlocking wall projections to prevent furtherseparation.

FIG. 9, FIG. 10, and FIG. 11 illustrate a bootless, sagittal planecutaway of an embodiment of the subject invention in flexion, normal andextension as shown in FIG. 6, FIG. 7, and FIG. 8. A spring element 9-1opposes the collapse of the superior hydraulic cylinder wall 6-1 overthe inferior hydraulic cylinder wall. In this embodiment, the twohydraulic cylinder wall elements slideably move along their matchingwall surfaces without interference, except for the wall projections orlock rings at the top edge of 4-1 and bottom edge of 6-1. The turningaxis for the sagittal rotation cylinder 2-1 is indicated by 16-1

FIG. 12 is a quadrant cutaway of a perspective view of an embodiment ofthe assembled invention, which exposes most of the invention's principleelements.

FIG. 13 shows a exploded side view of an embodiment of the subjectinvention without the boot, illustrating most, but not all, of theinvention elements, from left to right, an inferior vertebral plate 1-1,a sagittal rotation cylinder 2-1, a spring platform and an inferiorhydraulic cylinder base 3-1, an inferior hydraulic cylinder wall 4-1, asuperior hydraulic cylinder wall 6-1, a top cover 10-1 to the superiorhydraulic cylinder wall, a lateral rotation cylinder 11-1, a superiorvertebral plate 12-1, and locking keys 13-1. In this embodiment, thelocking keys, which can press fit or weld into the inferior vertebralplate 1-1 are not shown, but the slot 1-1-5 into which the keys fit isillustrated. A sagittal rotation and slider axis 16-1 (out of the page),a lateral rotation and slider axis 17-1, and an axial rotation andslider axis 18-1 are also shown.

FIG. 14 is a perspective exploded view of an embodiment of the inventionassembly without the boot. More details of the corresponding inventionelements come into view. In particular, a spring stack element 9-1, aninferior segmented-wall cylinder core 5-1, and a superior segmented-wallcylinder core 7-1.

FIG. 15 shows, in exploded perspective view, all the elements of aparticular embodiment of the subject invention except the boot, atoroidal belt, and various cushioning structures that can be used withinthe mechanism.

FIG. 16 is a perspective view of an embodiment of the subject inventionfrom the top of the plate with a quadrant cut out that allows one toobserve the different structures and surfaces of the superior vertebralplate.

FIG. 17 details a top cutaway perspective view of an embodiment of alateral rotation cylinder of the subject invention. A sagittal rotationcylinder oriented the same way (curved cylinder surface up) would appearthe same in this embodiment. This embodiment shows how the lock key canfit into an oversized slot 11-1-5 in the cylinder. The slot is slightlylarger than the key and the curvatures for both the top key surface13-1-1 and underside key surface 13-1-2 match the slot surfaces 11-1-11and 11-1-10. The center of curvature for all these surfaces falls on thecylinder's turning axis. This arrangement allows the lateral rotationcylinder to rotate freely about the keys located at each end. At thesame time, the keys can retain the lateral rotation cylinder within thesocket of the superior vertebral plate 12-1. The key slot 11-1-5 can beoversized in degrees by twice the desired maximum rotation angle of thecylinder. Also shown are hydraulic portals 11-1-3 that can drain thecentral hydraulic cylinder and provide lubricating fluid, underpressure, to the joint.

FIG. 18 details an embodiment of a superior hydraulic cylinder wall 6-1and its superior inner core 7-1, the latter consisting of inner cylinder7-1-1 and walled segments 7-1-2. Also seen are the top cover 10-1 andsome of its features.

FIG. 19 details an embodiment of an inferior hydraulic cylinder wall4-1, its interior core 5-1, the walled segments 5-1-2 of the inferiorinner core.

FIG. 20 illustrates various features of an embodiment of the spring base3-1 that can be utilized with an embodiment of the subject invention,which, in this embodiment, simultaneously serves as the inferiorhydraulic cylinder wall base and spring element platform.

FIG. 21 shows the undersurface features of the top cover 10-1 of thesuperior hydraulic cylinder wall. In this embodiment, the hydraulicportals 10-1-3 have a 90 degree rotation to similar elements 3-1-3 andthe indented wall-slots 10-1-4 have a β+2α rotation with respect tosimilar element 3-1-4 on the base 3-1. This arrangement of wall slotsallows the walled segments of the inferior (superior) inner core toslide into the wall-slots of element 10-1 (3-1) without interference.The walls subtend an angle β and the maximum angle of rotation about theaxial axis through the center of the top cover and spring base equals2α, or ±α.

FIG. 22 illustrates the principal features of the sagittal rotationcylinder, which, in one embodiment, is identical in structure to thelateral rotation cylinder. This view shows what would be the other sideof the lateral rotation cylinder in FIG. 17. The concavity 2-1-6 is abearing raceway with bearing stops 2-1-12. These bearing stops support acurvate concavity 2-1-5 through which the cylinder can rotate freelyabout conforming surfaces on the lock keys.

FIG. 23 shows how the sagittal rotation cylinder 2-1, the bearing stop2-1-12 and the lock key 13-1 fit together in an embodiment of thesubject invention. The lock keys at each end of the sagittal rotationcylinder can press fit or weld into the inferior vertebral plate 1-1,but fit loosely into the bearing stops 2-1-12 so as not to bind orhinder rotation of the sagittal cylinder about the keys or to bear anyload, which is transmitted through the larger surface 2-1-1 and 2-1-2 ofthe cylinder. At the same time, the keys retain the sagittal rotationcylinder within the socket of the inferior vertebral plate 1-1.

FIG. 24 is a perspective end-view of the sagittal rotation cylindershown in FIG. 23. In this embodiment the lock key 13-1 has 2γ degrees ofclearance in the bearing stop slot 2-1-5, permitting ±γ degrees ofrotation of the cylinder about the lock key. Only the end surface 13-1-4of lock key is visible from this perspective. The rotational jointformed by the lock key and the bearing stop forms a lower order pair.The axis of rotation 16-1 of the cylinder is out of the page.

FIG. 25 illustrates how the superior segmented-wall cylinder 7-1 and theinferior segmented-wall cylinder 5-1, of an embodiment of the subjectinvention shown above, mesh together as the superior and inferiorhydraulic cylinder walls start from full flexion a) to neutral b) andthen full extension c). Stand alone drawings of 7-1 and 5-1 appear inFIG. 18 and FIG. 19. In a particular embodiment, the walled segments5-1-2 extend into slots 10-1-4 of the top plate 10-1 of the superiorwhen the superior hydraulic cylinder wall completely closes, as in thefull flexion position of the FSU as shown in a). Similarly, the walledsegments 7-1-2 extend into slots 3-1-4 of the spring base 3-1. Atmaximum compression and maximum extension, and all positions in between,there exists a central core of support within the inside diameter of thespring elements. The overlap in the segmented walls 7-1-2 and 5-1-2assures this feature and that overlap requires that retaining cavities3-1-4 and 10-1-4 be cut into plates 3-1 and 10-1 to accommodate fullcompression. Axial rotation is still permitted, even when the wallsegments penetrate into cavities 3-1-4 and 10-1-4, as those cavities areoversized to accommodate the permitted axial rotation.

FIG. 26 shows one embodiment of the spring stack 9-1 using Bellevillesprings 9-1-1 in a series stack with guard rings 9-1-2 and 9-1-3. Theguard rings keep series of approximately matched-pairs of Bellevillesprings from inverting to parallel configuration under full compression.

FIG. 27 demonstrates another embodiment of the subject inventionutilizing a modified Belleville spring 9-2-1 comprising raised lips9-2-2 on the edge shown. In this embodiment, when series springconfigurations use approximately matched-pairs, the edges preventcomplete closure under full compression and, hence, prevent inversion ofa washer in the pair. The edges act in the same manner as the guardrings of the previous figure.

FIG. 28 provides a quadrant, perspective cutaway view to illustrate thevarious features and surfaces of an embodiment of the inferior vertebralplate 1-1. In a further embodiment, the superior vertebral plateunderside matches the topside shown here. However, the sagittal cylindersocket 1-1-10 and 1-1-11 is oriented at 90 degrees to lateral cylindersocket 12-1-10 and 12-1-11 in the superior vertebral plate 12-1 (seeFIG. 16).

FIG. 29 illustrates a quadrant cutaway perspective of a secondembodiment of the subject invention, without a boot, that uses ballbearings in place of lower order pairs for the joints. The partnumbering scheme can show the functional relationships between the twoembodiments. Often, for example, n-2 or n-2-m corresponds functionally,to elements n-1 or n-1-m. There are exceptions, but this observationwill make understanding the second embodiment easier, if the firstembodiment is understood.

FIG. 30 is a perspective exploded view of the assembly of thealternative embodiment, again, without the boot. The various bearingraceways can be seen here without the ball bearings. Seen are theprinciple elements of this embodiment: the inferior vertebral plate 1-2,the sagittal rotation cylinder 2-2, the spring base plate 3-2, theinferior segmented-wall cylinder 5-2, the spring stack 9-1, the inferiorhydraulic cylinder wall 4-2, the superior hydraulic cylinder wall 6-2,the superior segmented-wall cylinder 7-2, the top plate 10-2 of thesuperior hydraulic cylinder wall, the lateral rotation cylinder 11-2,and the superior vertebral plate 12-2.

FIG. 31 shows the embodiment wherein the sagittal rotation cylinder 2-2incorporates ball bearings and ball bearing raceways that mesh with theother half of the raceways embedded into the inferior vertebral plate1-2. The rotational ball bearings are not shown so as to illustrate theraceways better. The sagittal slider ball bearings 19-1, however, areshown. In a further embodiment, the lateral rotation cylinder 11-2 isidentical to the sagittal rotation cylinder 2-2 in structure.

FIG. 32 shows the top features of the sagittal rotation cylinder 2-2. Inthis embodiment, on the sagittal bearing structure 2-2-3, theprojections 2-2-18 and 2-2-19 can be replaced with bearing rods. Thegrooves 2-2-5 provide clearance for bearing rods in the sagittal sliderraceway. Refer to bearing rods 10-2-5 of the lateral cylindrical jointconfiguration (FIG. 36) for one embodiment of how this is done for thesagittal cylindrical joint. The sagittal slider bearing stops 2-2-10 andbearing separators 2-2-20 can hold the bearings 19-1 in place and can bepress fit or otherwise fixed into place. The sagittal slider bearingstructure translates parallel to the rotation axis 16-1 within aconcavity on the inferior surface of the spring platform base 3-2 FIG.33 details features of the sagittal linear bearing stops 2-2-10 andbearing separators 2-2-20 of the alternative embodiment.

FIG. 34 illustrates the alternative embodiment wherein the sagittalrotation cylinder with the sagittal slider bearings and bearing stopsare removed to expose the bearing raceway concavity 2-2-28 cut from thesagittal linear bearing structure 2-2-3. The rotational bearings havealso been removed in this view to show the rotational bearing chambers.

FIG. 35 depicts the alternative embodiment with the assembly of thecentral hydraulic cylinder with the lateral and sagittal rotationalcylinders mounted. Constituent elements that can be used with thisembodiment are the lateral rotation cylinder 11-2, the top plate 10-2,the superior hydraulic cylinder wall 6-2, the inferior hydrauliccylinder wall 4-2, the spring platform base 3-2, and the sagittalrotation cylinder 2-2. In a further embodiment, the ball bearings of theprismatic joint slideably lock 11-2 and 10-2 together. The bearing plugs10-2-4 can be press-fit or otherwise fixed into place at each end, 11-2and 10-2 do not separate during lateral translation of 11-2 with respectto the subassembly below it. Element 12-2, not shown in this diagram, isfixed to 11-2 with respect to any relative translation between 11-2 and12-2, but 12-2 is free to rotate about cylinder 11-2. Element 1-2, notshown in this diagram, is fixed to 2-2 with respect to any relativetranslation between 1-2 and 2-2, but 1-2 is free to rotate aboutcylinder 2-2.

FIG. 36 shows a view that exposes the interior of the superior hydrauliccylinder and the lateral slider bearing raceway in the top plate 10-2,of the alternative embodiment. In this particular embodiment, ballbearings (not shown) rest on the rods 10-2-5 and lock 10-2 and 11-2together, after the later is slid into position and blocked at eitherend with prismatic bearing stops 10-2-4. In a further particularembodiment, the superior hydraulic cylinder wall 6-2 and superiorsegmented-wall cylinder 7-2 weld or fix to the top plate 10-2. Inanother embodiment, 10-2, 7-2, and 6-2 can be produced as a single unitor, in another method, 7-2, as a separate unit, and can be centered andwelded or otherwise fixed to an integrated, or molded, 10-2 combinedwith 6-2.

FIG. 37 provides an exploded, perspective view of a subassembly of thesubject alternative embodiment having the inferior hydraulic cylinderwall 4-2, the inferior segmented-wall cylinder 5-2, the spring platform3-2, and the sagittal rotation cylinder 2-2 with attached sagittalprismatic ball bearings inserted with ball bearing separators and stops.In this embodiment, element 2-2 slides into the slot provided in 3-2 andlocks into place by press fit of prismatic bearing blocks 3-2-4 at eachend of 3-2. Elements 4-2 and 5-2 can be welded or fixed to 3-2 or cut ormolded as an integrated part with 3-2.

FIG. 38 provides a quadrant cutaway perspective of an embodiment of theelements 1-2, 2-2 and a split of an embodiment of elements 3-2, and 5-2.In this embodiment, the rods and the ball bearings that slide alongthem, retain 2-2 within 1-2, once the bearing stops 2-26 and 2-27 thatfit in the curvate bearing raceways of 2-2 and the bearing stops 1-2-15and 1-2-19 that fit in the curvate bearing raceways of 1-2 (see FIG. 39)have been inserted.

FIG. 39 shows in the alternative embodiment, the curvate bearing rods2-22, 2-23, 2-24, and 2-25, bearing stops 2-26, and 2-27, and theirmirror images in the sagittal plane passing through the central axis ofthe subject invention, that fit into the raceways of 2-2. The curvatebearing rods 2-22, 2-23, 2-24, and 2-25, bearing separators 1-2-16,bearing stops 1-2-20 and 1-2-19 and their mirror images in the sagittalplane passing through the central axis of the device, fit into theraceways of 1-2.

FIG. 40 is a view of the frontal plane section of the inferior vertebralplate 1-2 and the sagittal rotation cylinder 2-2 through the centralaxis of an alternative embodiment of the invention. The section showshow steel ball bearings suspend on four rails (for example, pair 1-2-17,1-2-18 and pair 2-22, 2-23). This arrangement can allow limited rotationof 2-2 with respect to 1-2 about the sagittal axis 16-1, while forcingthe two elements 1-2 and 2-2 to move together without relative motionfor all other directions.

FIG. 41 details sagittal rotation bearing elements at one end of 2-2 anembodiment of the subject invention. The bearing elements at the otherend are a mirror images of this one about a frontal plane through thecenter axis of the device. These end bearing elements can have center ofcurvature on the axis 16-1, but have different dimensions than theinside bearing elements (FIG. 43 and FIG. 44) as they fit at an angle(see FIG. 40)

FIG. 42 provides a partially exploded view of an alternative embodimentshowing the end bearings of 2-2.

FIG. 43 shows details of sagittal rotation bearing elements for theinside raceways of 2-2 of an alternative embodiment of the subjectinvention. A mirror image of these bearing elements about a frontalplane through the center axis of the device, fit into the other insideraceway

FIG. 44 provides a partially exploded view of the inner bearing elementsof 2-2.

FIG. 45 shows an embodiment of a rod bearing that separates the bearingstops 2-26 from the bearing rods 2-2-22 and 2-2-23. The bearing stopsfixedly join the rods into a unit, at least at one end. The other endcan be movable and the rods insert into them with a press fit.

FIG. 46 shows a curvate rod bearing 19-2 embodiment that allows thecurvate bearing rods (1-2-17, 1-2-18, 2-2-22, 2-2-23) to slide by as 1-2rotates relative to 2-2.

FIG. 47 shows a quadrant cutaway perspective view of 1-2 and the wholeelement 2-2 depicting surface features of an embodiment of a sagittalcylindrical socket in 1-2 along with the embedded element 2-2.

FIG. 48 displays an embodiment of the entire lateral rotation cylinder11-2 embedded into a perspective half of the superior vertebral plate12-2.

FIG. 49 details the socket surfaces in 1-2 which can be hidden by thecylinder 2-2.

FIG. 50 shows a perspective, frontal plane cutaway view through themiddle of 1-2 and 2-2, but with the bearing elements removed so thebearing pathways can be seen clearly.

FIG. 51 illustrates a perspective view of an embodiment of a toroid beltcushion 22-2 that can be utilized with embodiments of the subjectinvention. In this view, the superior vertebral plate 12-2 and boot 8-1have been removed and a quadrant of the inferior vertebral plane 1-2excised. The toroid belt can assist with cushioning off-axial axiscompressive motions of the superior vertebral plate with respect to theinferior vertebral plate and helps relieve shear stresses.

FIG. 52 depicts a toroid belt cushion element 22-2 of FIG. 5 in aquadrant cutaway perspective of the entire device with the boot removed.

FIG. 53 shows a perspective view of a further alternative embodiment ofthe inferior and superior hydraulic cylinder shells 4-3 and 6-3. In aparticular embodiment, shell 4-3 has three 60 degree, thick-wallsegments 4-3-1 and three 60 degree, thin-wall segments 4-3-4, with guardring segments 4-3-2 for each of the three thin-wall segments. Shell 6-3can have a single thin-wall segment 6-3-1, with guard ring segments6-3-2 associated with, and conforming to, each of the guard rings 4-3-2at the top of each of the three thin-wall segments 4-3-4 of the inferiorhydraulic cylinder shell. The thick-walled segments 4-3-1 of shell 4-3add greater strength to the central hydraulic cylinder, and example ofwhich is shown in FIG. 35 for the higher order bearing version.

DETAILED DISCLOSURE

The subject invention provides embodiments of intervertebral diskprotheses. More specifically, the subject invention pertains to one ormore embodiments of an intervertebral disk prosthesis capable ofproviding up to 6 degrees of freedom.

The subject invention is particularly useful for the treatment of spinaldisk herniation. However, a person with skill in the art will be able torecognize numerous other uses that would be applicable to the devicesand methods of the subject invention. Thus, while the subjectapplication describes a use for treatment and/or removal of spinal diskherniation, other modifications apparent to a person with skill in theart and having benefit of the subject disclosure are contemplated to bewithin the scope of the present invention.

Throughout the subject application, reference is made to a “firstembodiment” and a “second embodiment”. These terms are used merely forliterary convenience to refer to two specific embodiments describedherein that illustrate the various features of the subject invention.For example, the first embodiment of the subject invention is describedas having kinematically chained or kinematically linked surface bearingsor contacts. The second embodiment of this invention is described ashaving the surface bearings replaced by kinematically chained orkinematically linked line rod or multi-point contact ball bearings. Aswill be described herein, features and elements of each embodiment canbe interchangeable. A person with skill in the art, having benefit ofthe subject disclosure, would be able to determine numerous alternativearrangements of the elements and/or components described herein, orequivalent alternative embodiments therefore. Thus, the subjectinvention is not limited to only the first and second embodimentsdisclosed herein.

As used in the subject application, “kinematic chain”, “kinematiclinkage”, and “kinematic connection” refer to a mechanical linkageinseparably connecting the components of the device of the subjectinvention. It is known to those with skill in the art that a ‘mechanicallinkage’ is a series of physical links connected with joints to form aclosed chain, or a series of closed chains. Thus, as will be describedherein, the components of the device of the subject invention areinseparably linked, such that the components can move relative to eachother, but do not become separated one from the other. That is, thecomponents of the device of the subject invention remain interconnectedor physically attached at all times to each other, and to the vertebraewhen installed in an FSU.

The term “patient” as used herein, describes an animal, includingmammals to which the systems and methods of the present invention areapplied. Mammalian species that can benefit from the disclosed systemsand methods include, but are not limited to, apes, chimpanzees,orangutans, humans, monkeys; domesticated animals (e.g., pets) such asdogs, cats, guinea pigs, hamsters; veterinary uses for large animalssuch as cattle, horses, goats, sheep; and any wild animal for veterinaryor tracking purposes.

The terms “surgeon” or “physician” as used in the subject applicationare merely for literary convenience. The terms should not be construedas limiting in any way. The devices, apparatuses, methods, techniquesand/or procedures of the subject invention could be utilized by anyperson desiring or needing to do so and having the necessary skill andunderstanding of the invention.

Also, as used herein, and unless otherwise specifically stated, theterms “operable communication” and “operably connected” mean that theparticular elements are connected in such a way that they cooperate toachieve their intended function or functions. The “connection” may bedirect, or indirect, physical or remote.

The present invention is more particularly described in the followingexamples that are intended to be illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art. As used in the specification and in the claims, the singularfor “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise.

With reference to the attached figures, which show certain embodimentsof the subject invention, it can be seen that the subject invention(shown generally in FIG. 1, FIG. 12, FIG. 14, FIG. 29 and FIG. 30)differs from existing designs. A novel implementation 1) allowssix-degrees-of-freedom throughout the Functional Spinal Unit (FSU)workspace while simultaneously bearing compressive, tension and torsionloads; 2) maintains the integrity of the variable intervertebral spacingrequired (under compression the intervertebral gap should narrow someand under tension widen some); and retains an unbroken (fully connected)mechanical linkage between the superior and inferior vertebra during anynormal motion of the affected FSU, the later can promote joint stabilityand assist in preventing spondylolisthesis of the FSU.

When appropriately scaled, the invention is capable of trackingarbitrary three-dimensional translational and three-dimensionalrotational motions of the superior vertebra with respect to the inferiorvertebra. In a patient, this can include an FSU from spinal discs C2-C3down to L5-S1, while adjusting the disc height and accommodating thevarious forces and moments-of-force applied to the FSU during anymotion. Thus, the subject invention can accommodate the workspace of anyFSU along the spine and is a considerable improvement over current discdesigns.

One to three cylindrical joints can kinematically permit motion from twoto six degrees of freedom throughout the workspace of the FSU. The rangeof motion for all revolute (0 to ±15 degrees of rotation) and slider (0to ±1.5 millimeters of displacement) joints in the invention can bemechanically programmed with judicious choice of joint limit stops,including cushioned stops to reduce impact wear on the stops. A centralhydraulic cylinder spring-dashpot system offers both static and dynamicstability to the FSU with shock absorbing characteristics. The centralhydraulic cylinder rotates and slides sagittally with respect to theinferior vertebra of an FSU and rotates and slides laterally withrespect to the superior vertebra of an FSU. The relative motion of thecentral hydraulic spring-dashpot with respect to the inferior andsuperior vertebrae of an FSU allows it to generate an opposing force toany compressive static or dynamic load acting on the rotating andsliding axial axis of the FSU, regardless of the position of thevertebrae and the complex motion involved. Non-axial components of theforce will act to move the prosthesis until joint limit stops rigidlyoppose any further motion in that particular direction or orientation. Aprotective boot assists in the hydraulic and shock absorptionproperties. Additional cushioning elements can also be used to enhanceshock absorption.

The cylindrical joint axes of the invention can parallel a rotatedversion of the sagittal, frontal and axial plane axes, as defined for ananimal body, enabling lateral, sagittal, and axial axes displacementsand lateral, sagittal and axial axes rotations. The actual inclinationof the invention with respect to the body coordinates depends upon thenatural inclination of the FSU to the body planes. Specifically, theinvention should be inserted into an FSU, with disc removed, such thatthe superior and inferior surfaces are parallel to the client's FSUvertebral surfaces in the normal posture. Such placement will maximizethe effective work space of the prosthesis.

In the description to follow, the axes of the cylindrical joints will belabeled as sagittal, lateral, and axial, it being understood that theseaxes are actually parallel to rotated versions of the typically definedpatient body axes and that the frontal, sagittal and axial planes in thetext will refer to those of the FSU body and not the animal body.

A particular embodiment of the spinal disc prosthesis of the subjectinvention is operated by the muscles and ligaments of the spine wheninstalled in an FSU. These muscles and ligaments drive thespring-damping system and resultant motion of the prosthesis. Thekinematic generality of the prosthesis's motion capabilities, allowsnatural movements of any FSU in which the prosthesis is placed. In oneembodiment, the three cylindrical joint axes (16-1, 17-1, 18-1, FIG. 13)are mutually orthogonal, providing three independent degrees of freedomfor rotation and translation along the sagittal cylindrical joint'sturning and sliding axis, referred to herein as the sagittal axis 16-1(FIG. 23), the lateral cylindrical joint turning and sliding axis,referred to herein as the lateral axis 17-1 (FIG. 17), and the centralaxis of the hydraulic cylinder, referred to herein as the axial axis18-1 (FIG. 18). In one embodiment, the sagittal axis 16-1 is fixed tothe inferior vertebral plate 1-1, hence, by rigid connection, to thesuperior vertebra of the FSU. In a further embodiment, the lateral axisis fixed to the superior vertebral plate 12-1, hence, by rigidconnection, to the superior vertebra of the FSU. In a yet furtherembodiment, the axial axis is always the common normal of the skewsagittal and lateral axes for arbitrary motions of the superior vertebraof the FSU with respect to its inferior vertebra.

The Instantaneous Axis of Rotation (IAR) of an FSU often changes duringthe motion of its superior vertebra with respect to its inferiorvertebra. As mentioned above, the orienting capability of the sagittal,lateral, and axial axes of rotation of the three revolute joints, whichis constrained to be within the device, and is not kinematicallysufficient to mimic natural motion of the FSU, but, the translationcapabilities of the three cylindrical joints correct this. In oneembodiment, to sufficiently kinematically mimic natural motion of an FSUand accommodate additional translational requirements, the subjectinvention is configured with three independent linear translations, oneassociated with each cylindrical joint, that can, when coupled with thethree cylindrical rotations, accommodate the differences indisplacements induced by a variable IAR. This embodiment of the subjectinvention provides the same motion capabilities of a moving IAR withoutneeding to duplicate the means by which the spine generates the FSUmotion.

In a specific embodiment of the subject invention, the three,spatially-independent, cylindrical joints form a kinematic chain, joinedtogether in continuous physical linkage that is inseparable at all timesand for all motions, that determines the location and orientation of thesuperior vertebra with respect to an inferior vertebra of an FSU. Thespinal disc prosthetic can constrain the relative motion of the superiorvertebra with respect to the inferior vertebra to its natural locus ofmotion and can maintain, through the load bearing spring and cushionelements, the correct variation in intervertebral spacing during motion(see FIGS. 6-11).

Advantageously, embodiments the subject application can provide 1)effective static load bearing through one or more spring elements, 2)hydraulic damping and shock absorption by means of hydraulic pumpingaction, 3) cushioning in the various joint axes conjoined with atorus-shaped, general-purpose cushion element, constrained within thedevice by a central cylindrical core, 4) automatic hydraulic lubricationof all joints, 5) intervertebral stability and inseparability throughmechanical linkage from superior to inferior vertebral plates thatprevents motion outside the normal, natural range, 6) mechanicallyprogrammable vertebral spacing under nominal compression load-bearing byappropriate selection of spring constants, height and number in thecentral spring element or stack, 7) 6-DOF motion tracking with variabledisc height throughout the prosthesis workspace, and 8) a mechanicallyprogrammable prosthesis workspace through judicious sizing of linear androtational joint stops. The degrees of rotation and millimeters oflinear translation allowed by the joint stops can be independentlyspecified for each cylindrical joint, enabling the invention to matchthe device workspace to that of the client's FSU workspace.

The motion elements of the prosthetic device of the subject inventioncan be fabricated of, for example, titanium steel,titanium-carbide-coated stainless steel, bio-inert hardened stainlesssteel, polyurethane, polyurethane thermoplastic,cobalt-chromium-molybdenum alloy, plastic, ceramics, glass, or othermaterials or combinations thereof. In a second embodiment, the motionelements of the prosthetic device of the subject invention can befabricated with hardened stainless steel ball-bearings and bearing rodsthat can move on hardened stainless steel curvate or linear rods thatfit into raceway cavities of the various titanium or plastic elements.In an alternative embodiment, a mix of polyurethane thermoplasticbearings and polyurethane, titanium, ceramics,cobalt-chromium-molybdenum alloy and titanium-carbide-coated hardenedstainless steel components can be utilized. The device of the subjectinvention allows for joint limits and stops on all degrees of freedom,which permits mechanical programming of its workspace to match the FSUworkspace. The invention can, thus, accommodate the wide variability ofFSU motion at different locations within the spine and between spines ofdifferent individuals.

In one embodiment, the modular 6-DOF spatial mechanism for spinal discprosthesis of the subject invention comprises a superior and an inferiorvertebral plate (12-1 and 1-1). In a further embodiment, the spinal discprosthesis of the subject invention comprises a flexible,boot-protected, modular and replaceable 6-DOF prosthetic disc mechanism(mechanical linkage). In one embodiment, the vertebral plates can beformed from a biocompatible material such as, for example, titanium,cobalt-chromium-molybdenum alloy, or titanium-carbide-coated stainlesssteel with a bone fusion matrix on the side of the plate shaped as aspherical surface to enhance surface area contact between vertebra andthe vertebral plate.

Any number of existing techniques known to those with skill in the artmay be used to embed the superior vertebral plate of the subjectinvention into the bone of the superior vertebra and the inferiorvertebral plate into the bone of the inferior vertebra of an FSU. It iscontemplated that such techniques are within the scope of the subjectinvention.

In a further embodiment, a flexible boot (8-1: FIGS. 1, 4, and 5)surrounds the prosthetic device of the subject invention. The boot canprovide a biocompatible impermeable barrier between fluids that may besealed within the prosthetic device and fluids within surroundingtissues, such as, for example, a silicone fluid biocompatible salinesolution. In one embodiment, the boot can consist of a sturdy, flexibleor elastic material, such as, for example, corrugated materials, wovenfiber materials, and elastic materials, or other non-homogeneousmaterials. In a further preferred embodiment, the boot comprises woven,flexible fibers embedded in a strong, flexible silicon elastomer thatcan block fluid transfer. The embedded fiber weave, in the embodimentmentioned above, can assist in torsion loading on the prosthesis as wellas loading during flexion and extension. In a further embodiment, theweave direction of the embedded fibers is diagonal relative to thecentral axis of a spherical or right-circular cylinder embodiment of theboot structure. In a particular embodiment, a corrugated boot,consisting of a rugged fiber elastomer designed for flexibility andtoughness, assists in torsion loading on all axes and opposes extensionunder nominal conditions, thus, reducing nominal spinal muscle stress inthe neutral position. In a further embodiment, one or more joint limitstops can be utilized on one or more of the rotational joints of theinvention to limit the amount of torsion the boot can experience,reducing the possibility of tears from overstress.

All displacements and rotations of the joints can be mechanicallyprogrammed to specific joint limits by appropriately installed jointstops. The joint stops can be rigid, or, to reduce wear, cushioned withmaterials falling within a wide range of durometer choices from 50 to100. For the ball-bearing embodiment the rotating joint stops areinserted into the bearing raceways.

In one embodiment, the corrugated boot has asymmetric thickness, usingmore reinforcing fiber in the posterior portion and less in the anteriorportion, making the anterior portion more flexible and the posteriorportion less flexible, but stronger and more durable. This configurationcan reduce interaction with the spinal column or nerve ganglia when theboot is expanding and/or contracting. For example, as the FSU flexes,the boot can contract, primarily the highly flexible thinner sections.In a neutral position of the FSU, the boot can be under slight tension.At maximum compression of the FSU, the boot can bulge from hydraulicpressure and expanding cushioning material inside the device; however,without those pressures the boot would be slack. At maximum extension,the boot stretches, from its neutral position. In one embodiment, atmaximum extension, the boot stretches an additional 20% in its anteriorportion and about 10% or less in the posterior.

A further embodiment utilizes a fibrous belt or toroidal tube 22-2(FIGS. 51 and 52) as an additional cushioning element in the subjectinvention to assist the boot and central cylindrical joint in resistingshocks and arbitrary FSU force loads by compressing sections of thetoroid. In one embodiment, the toroidal tube 22-2 is filled withcompressible fluids or gels, such as, for example, hydrophilic gels orhydrogels. In an alternative embodiment, the toroidal tube 22-2 is asolid material. This belt can wrap around the central hydraulic cylinderelements and be confined to the disc volume by same. In a furtherembodiment, a toroidal belt or tube can float, not be fixed to any otherelements. In an alternative embodiment, a toroidal belt can attached oneor both of the vertebral plates. In a still further alternativeembodiment, a toroidal belt can be integrated into the boot. With thisembodiment, the boot and toroidal belt resembles the concept of fiberring and cushion element as instructed by Casutt (U.S. Pat. No.6,645,248B2). In one embodiment of the subject invention, the toroidalbelt moves about with the central hydraulic cylinder as the cylindertranslates axially, laterally or sagittally with respect to thevertebral plates. The toroidal belt can oppose the cylinder motion inall cases since it either pushes against the boot when not fixed to anyelements or, if fixed to the vertebral plates, the belt pulls againstthose plates. Effectively, in either case, the toroidal belt can providea three-dimensional, universal resilient or spring-like action opposingthe cylinder motion. The belt can also strengthen the central hydrauliccylinder walls to provide additional sheer stress tolerance for thedevice.

In a further embodiment, a lubricating fluid is contained within theprosthetic device of the subject invention by the impermeable boot seal.The lubricating fluid can be pumped through fluid portals, or otherwisemoved around the elements of the device, by the piston action of thesuperior and inferior hydraulic cylinders during spinal motion. Thesecylinders can further contain spring 9-1 and cushioning elements 15-1(FIG. 11) to provide a spring-dashpot action during FSU motion.

In one embodiment, the spring-dashpot element of the central cylindricaljoint element consists of superior and inferior external walls thatslide over one another in a telescoping manner with retaining rings thatprevent separation. The external walls enclose a cavity that can becylindrical in shape. The superior and inferior external walls can havecorresponding segmented-wall inner cores that slide past one another asthe external walls slide to and fro. The inner segmented walls meshwithout interference with one another. Each gap in the superior innercore wall is matched by a solid wall segment in the inferior wall, andvice versa. The preferred number of segments can be three or more and becut from a solid cylindrical shape with a partial cavity, thus, formingsupporting inner center posts to the segmented walls. The center postsof the inner core elements, top and bottom, can partially consist ofcushion elements mounted on rigid elements to further promote shockabsorption. The external walls and the segmented-wall internal corescan, together, firmly hold one or more spring elements in place, forexample, a stack of Bellville springs, in a variety of series/parallelspring configurations within the available cavity space of the centralhydraulic cylinder. The number, arrangement and spring rates of theBelleville springs in the stack will determine the intervertebralspacing when the invention is under load in the spine. This means theinvention can accommodate a wide variety of practical situations by thesimple expedient of changing the composition of the spring stack duringmanufacture, leading to easily produced different model numbers. In thisway, the invention can compensate for client needs without changing thedesign and structure of any of the invention elements. In effect, in thepreferred embodiment, only the spring stack composition changes for awide range of models.

The walls of the superior and inferior cylindrical elements, along withthe spring, can constitute a spring-dashpot shock absorbing system.Hydraulic portals within the device can facilitate shock absorbingcharacteristic while at the same time forcing a bio-lubricant to flowthrough and around the components of the bearing interfaces of thedevice. The combined dual cylinder and the spring stack provide a columnelement that resists shear forces and promotes the rotation andtranslation of the various joint elements when the FSU is subjected toshear forces.

In one embodiment, the inferior hydraulic cylinder telescopes in and outof the superior hydraulic cylinder during flexion and extension. Lateraland other motions can also affect the amount of telescoping, whichaccommodates, up to joint limits, the natural dictates of the FSUmotion. In a further embodiment, the superior and inferior hydrauliccylinders have guard-rings or edges (4-1-2, FIG. 19 and 6-1-2, FIG. 18)to keep them from separating at maximum extension, thus preserving themechanical linkage or inseparable connection between the vertebrae ofthe FSU. In a still further embodiment, the guard-rings or edges do nothave to extend all the way around the cylinder, but can be segmented tooccupy, only every other 60 degree arc around the cylinder wall. Thisallows the cylinder walls to be thicker and stronger in those 60 degreesegments where there is no edge, guard-ring, or bearing.

FIG. 53 shows a perspective view of an embodiment of the inferior 4-3and superior 6-3 hydraulic cylinder shells. In this embodiment, 4-3 hasthree 60 degree, thick-wall segments 4-3-1 and three 60 degree,thin-wall segments 4-3-4, with guard ring segments 4-3-2 for each of thethree thin-wall segments. Shell 6-3 has only a single thin-wall segment6-3-1, with guard ring segments 6-3-2 associated with, and conformingto, each of the guard rings 4-3-2 at the top of each of the threethin-wall segments 4-3-4 of the inferior hydraulic cylinder shell. Thethick-walled segments 4-3-1 of shell 4-3 can add greater strength to thecentral hydraulic cylinder while still providing axial slider jointlimits using the guard ring segments 4-3-2 in the three 60 degreesegments 4-3-4 that have matching guard ring segments 6-3-2 at base of6-3. In this arrangement there is no sacrifice in the wall thickness of6-3. The compromise is that the guard rings span only 180 degrees of thewalls circumference. This should not affect the joint limit stopfunction. In a further embodiment, the guard rings can be approximately¼ to ½ the thickness of a wall segment.

In a further embodiment of the subject invention, hydraulic portals3-1-3, 4-1-3, 6-1-3, and 10-1-3 (FIGS. 13 and 19) through the varioussurfaces allow transfer of fluid into and out of the telescopingcylinders. The hydraulic fluid, which can be pumped under pressure bythe natural action of spinal flexion and extension, tends to separateall the interacting bearing surfaces in a manner similar to the actionof synovial fluid in a diarthrodial joint; this can increase theefficiency of the bearing surface and reduce wear. Such fluids caninclude, but are not limited to a biocompatible silicone fluid, abiocompatible saline solution, oils of various types and viscosities,water, gels, other viscous materials, or combinations thereof.

The subject invention provides a spinal disc prosthesis (FIGS. 2, 14,and 30) capable of providing spatial movement with up to 6 independentdegrees of freedom. The modular prosthetic disc of the subject inventioncontains the mechanisms responsible for its general motion capability.In a further embodiment, the prosthesis can be surrounded by animpermeable boot 8-1 consisting of a resilient, fiber-reinforcedelastomer matrix that firmly attaches to the superior and inferiorvertebral plates 12-1 and 1-1. In a particular embodiment, the bootfiber weave is diamond shaped and can be woven into cylindrical orspherical 8-1-3 form (FIG. 5) or in bellows form (not shown). Varioustypes of reinforcing materials, weave type, and the fiber properties,along with mixed fibers can be used to construct the boot, much as tiremaking in the automotive industry. The boot structures can further havereinforcing rings 8-1-2 at the top and bottom. These rings withsurrounding fiber and elastomer attach fixedly to 12-1 and 1-1 in anynumber of methods known to a person with skill in the art. In aparticular embodiment, a channel 12-1-6 and 1-1-6 can be configured toclamp onto the rings, so the boot can withstand large forces withouttearing the boot matrix or pulling boot away from the device. The bootcan be shaped as in 8-1-1, to provide addition cushioning effect. Theboot can also block out bio-debris that might reduce joint mobility.

In a specific embodiment, principal mechanisms of the subject inventionconsists of three, spatially-independent cylindrical joints, for generalpositioning and orienting in three-dimensional space, that establish aninseparable kinematic chain or kinematic linkage between a superiorvertebral plate (12-1, FIG. 16) and an inferior vertebral plate (1-1,FIG. 28). The inferior vertebral plate 1-1 and superior vertebral plate12-1, in an embodiment, can be identical. More specifically, 1-1 can be3-dimensionally congruent to 12-1, i.e. there exists a rigid bodytransformation that will allow one to superimpose 1-1 onto 12-1. Incertain embodiments described herein, a statement regarding the featuresof one, therefore, applies to the other in such an embodiment. Forexample, surfaces 12-1-1, 12-1-2, 12-1-3, and 12-1-4 can be constructedto enhance bone fusion to 12-1. This observation also applies tocorresponding surfaces (not labeled) of 1-1.

In a particular embodiment, each cylindrical joint provides oneindependent rotational and one independent linear translational degreeof freedom with mechanically programmable joint stops and the means forload bearing elements for each degree of freedom. The sagittal rotationcylinder 2-1 (FIG. 22) and the lateral rotation cylinder 11-1 (FIG. 17),appear in context in FIGS. 13, 14, and 15. The third cylindrical joint,also referred to herein as the central hydraulic cylinder, can beconfigured from the inferior hydraulic cylinder, consisting of elements3-1, 4-1, and 5-1, and the superior hydraulic cylinder, consisting ofelements 6-1, 7-1, and 10-1.

In a further embodiment, the lateral rotation cylinder 11-1 (FIG. 17)fits into a conforming cavity in the superior vertebral plate 12-1 (FIG.16). In a still further embodiment, the cylindrical cavity 12-1-10conforms to the cylindrical surface 11-1-1 and the spherical cavities12-1-11 at each end conforms to the spherical surfaces 11-1-2 at eachend of 11-1. These cavities and surfaces can all have centers ofrotation on the lateral axis 17-1. As mentioned above, hydraulic portals11-1-3 pass lubricating fluid under pressure to the joint surfaces.

In another embodiment, the surfaces 11-1-4 at each end of cylinder 11-1are matched by surfaces at the end of the superior vertebral plate'scylinder cavity, planar in a preferred embodiment. A curvate slot 12-1-5allows press-fit insertion of lock keys 13-1 through plate 12-1. The keylength is such that it projects into the slightly oversized curvatecavity 11-1-5 of the lateral cylinder raceway plug 11-1-12. The surface13-1-4 at the other end of the key conforms to the surface 12-1-12 ofthe superior vertebral plate.

The superior hydraulic cylinder plate 10-1 (FIG. 18), using slidebearing block 10-1-2 fixed to its top and with locking projection10-1-5, can be slid into the lateral raceway 11-1-6 (FIG. 18), wherelocking edge cavities 11-1-7 can conform to projections 11-1-5, forminga lateral prismatic joint with plus or minus displacement 20-1 (FIG.12). A corresponding displacement can be applied to the sagittalprismatic joint. The latter is not shown for this embodiment, but thesagittal prismatic displacement 20-2 of the ball-bearing embodiment,FIG. 29, demonstrates the idea. After sliding 11-1 onto an alreadyassembled central hydraulic cylinder by means of bearing block 10-1-2fixed on top of plate 10-1 and inserting the subassembly into thelateral rotation socket of 12-1 described earlier, assembly can proceedwith positioning the lock keys 13-1 through the superior vertebral plateslot 12-1-5 and loosely into the oversized cavities 11-1-5 at either endof the lateral rotation cylinder. In one embodiment, the lock keys arepress-fit into position. The slots 11-1-5 allow the lateral cylinder torotate about the keys that project into the cylinder ends. An end-on,perspective view is shown for the sagittal rotation cylinder in FIG. 24.In one embodiment, the maximum value of angle γ can be 15 degrees. Theactual size of γ dictates the maximum rotation joint range, namely, 2γdegrees. Shims can be placed in the slot on either side of the centerposition to limit rotation to less than the maximum for that side.Further, the range about center can be unequal by appropriate shims. Asimilar observation applies to the lateral rotation cylinder.

After the lateral rotation cylinder 11-1 mates with the centralhydraulic cylinder, but before the lock keys 13-1 are positioned in slot12-1-5, both ends of the lateral rotation cylinder 11-1 can be sealedwith raceway plugs 11-1-12 into cavities 11-1-5 with upper curvatesurface 11-1-11 and lower curvate surface 11-1-10. In one embodiment,the raceway plugs 11-1-12 are inserted in the ends of the lateralrotation cylinder 11-1 by press fitting. In another embodiment, uppercurvate surface 11-1-11 and lower curvate surface 11-1-10 arecylindrical with the center of curvature on the lateral axis 17-1.Flange 11-1-13 on the plug can increase surface area contact with thebearing raceway cavity 11-1-6. Further, top curvate surface 11-1-1 canconform to curvate surface 13-1-1 as can 11-1-10 and 13-1-2, allowingthe cylinder to rotate about the lock keys from the common centers onthe lateral axis 17-1. The planar surfaces 13-1-2 of the keys conform tothose found at the sides of cavity 11-1-5. In a further embodiment, thelength d of plug 11-1-12 (shown for the corresponding sagittal plug2-1-12 in FIG. 22) fixes the displacement 20-1, allowing mechanicalprogramming of the displacement using different length plugs. In a stillfurther embodiment, a parameter e of the plug fixes the length of thecavity 11-1-5 (only shown for the sagittal plug 2-1-12 and its cavity2-1-5, FIG. 22).

In yet another embodiment, a sagittal rotation cylinder 2-1 (FIGS. 22and 23) provides similar features and functions for sagittal rotationand translation as the lateral rotation cylinder 11-1 does for lateralrotations and translations. In this embodiment, the sagittal rotationcylinder 2-1 fits into a conforming cavity in the inferior vertebralplate 1-1 (FIG. 28). In a further embodiment, the cylindrical cavity1-1-10 conforms to the cylindrical surface 2-1-3 and the sphericalcavities 1-1-11 at each end conforms to the spherical surfaces 2-1-2 ateach end of 2-1. In a still further embodiment, these cavities andsurfaces all have centers of rotation on the sagittal axis 16-1.

In another embodiment, the surfaces 2-1-4 at each end of cylinder 2-1arc matched by conforming surfaces at the end of the inferior vertebralplate cylinder cavity, planar in a preferred embodiment. A curvate slot1-1-5 allows positioning of lock keys 13-1 through plate 1-1. In oneembodiment, the lock keys are press-fit into the curvate slot. The keylength can be such that it projects into the slightly oversized curvatecavity 2-1-5 of the sagittal cylinder raceway plug 2-1-12. The surface13-1-4 at the other end of the key can also conform to the surface1-1-12 of the superior vertebral plate.

By way of a non-limiting example, the superior hydraulic cylinder plate3-1, can utilize slide bearing block 3-1-2 fixed to its top and withlocking projection 3-1-5, and slide into the sagittal raceway 2-1-6,where locking edge cavities 2-1-7 can further conform to projections3-1-5, forming a sagittal prismatic joint whose displacement equals thelength of 2-1 minus twice the length d of plug 2-1-12. After sliding 2-1onto an already assembled central hydraulic cylinder by means of plate3-1 and inserting the subassembly into the sagittal rotation socket of1-1 described earlier, assembly can proceed with positioning of the lockkeys 13-1 through the superior vertebral plate slot 1-1-5 and looselyinto the oversized cavities 2-1-5 at either end of the sagittal rotationcylinder 2-1. The slots 2-1-5 allow the sagittal cylinder to rotateabout the keys that project into the cylinder ends. An end-on,perspective view is shown in FIG. 24. The angle γ allows the cylinder torotate ±γ degrees about the sagittal rotation axis 16-1. In oneembodiment, the maximum value of angle γ can be 15 degrees. Slots withsmaller values 15>γ>0 can be used in a particular embodiment. The actualsize of 7 dictates the maximum rotation joint range, namely, 27 degrees.Shims can be placed in the slot on either side of the center position tolimit rotation to less than the maximum for that side. Further, therange about center can be made unequal by inserting more shims on oneside of the slot than on the other side. After the sagittal rotationcylinder 2-1 mates with the central hydraulic cylinder, but before thelock keys 13-1 are positioned into 1-1-5, both ends of the sagittalrotation cylinder 2-1 can be sealed with raceway plugs 2-1-12 intocavities 2-1-5 with upper curvate surface 2-1-11 and lower curvatesurface 2-1-10. In one embodiment, these surfaces are cylindrical, withcenter of curvature on the sagittal axis 16-1. In addition, flange2-1-13 on the plug can increase surface area contact with the bearingraceway cavity 2-1-6. Curvate surface 2-1-11 can conform to curvatesurface 13-1-1 as can 2-1-10 and 13-1-2, allowing the cylinder to rotateabout the lock keys from the common centers on the sagittal axis 16-1.The planar surfaces 13-1-2 of the keys can also conform to those foundat the sides of cavity 2-1-5. The lengths d of bearing plug or stop2-1-12 in FIG. 22 can fix the sagittal slider displacement, allowingmechanical programming of the displacement using different length plugs.A parameter e of the plug cans also fix the length of the cavity 2-1-5.

A spring element, in a one embodiment, is a series configured springstack (9-1) of up to 10 Belleville springs (9-1-1), loosely fit (FIGS.12 and 14) within the central hydraulic cylinder formed from superiorelements 10-3, 6-1, and 7-1 and inferior elements 3-1, 4-1, and 5-1. Ina particular embodiment, the Belleville springs are of a bio-inertmaterial. Other embodiments can configure combinations of series andparallel stacks with varying spring constants to provide non-linearspring performance when the springs are allowed to saturate, i.e. reachtheir maximum allowed deflection and, thus, operate the stack out of itslinear range. For example, as Belleville springs with smaller springconstants reach maximum deflection, the overall spring constant willincrease.

In a further embodiment, the segmented-wall inner cores 5-1 (FIG. 19)and 7-1 (FIG. 18) loosely fit into inner circular opening of thesprings, allowing the cores to slide by the springs during operationwhile providing an intact central shaft to stabilize and hold the springelements at all times and for all device configurations. The core wallsegments 5-1-2 and 7-1-2 can form a central column to help stabilize thesprings through all nominal movements of the FSU as seen in FIGS. 9-11and 25. In this embodiment, the segmented walls slide past one anotherwithout hindrance, and, at full compression, mesh as shown in FIG. 25.The wall segments 5-1-2 that extends past core 7-1-1 are capable offitting into slots 10-1-4 of plate 10-1. The wall segments 7-1-2 thatextends past core 5-1-1 are also capable of fitting into slots 3-1-4 ofplate 3-1. In one embodiment, slots 3-1-4 and 10-1-4 subtend angle β+2αin order to allow axial rotation of ±α, even in the fully compressedposition. The central elements 5-1-1 and 7-1-1 strengthen the innercores and can assist in resisting shear forces. The segmented wallssubtend β degrees and the gaps β+2α degrees (FIGS. 18 and 19). If 2nequals the number of walls, then preferred embodiments satisfy2n(β+α)=360 degrees. For preferred embodiments n=3 or 4, but otherchoices are possible. In an example embodiment n=3, 2n=6 and β+α=60degrees. For an axial rotation specification of ±5 degrees, α=5 degrees,β=55 degrees. For an axial rotation specification of ±10, α=10 degrees,β=50 degrees. The wall segments 5-1-2 and 7-1-2 can also act as jointstops and limit axial rotations to ±α degrees, ±5 up to ±10 degrees inone embodiment.

In one embodiment, the outer walls 4-1 of the inferior hydrauliccylinder and 6-1 of the superior hydraulic cylinder have conformingcurvate shapes 4-1-1 and 6-1-1, which can also help stabilize the springstack. In a further embodiment, seen for example, in FIG. 15, the outerwall curvate shape is cylindrical. In one embodiment, sufficient spaceis provided between 4-1, 6-1, and the springs to allow them to becompressed as much as 100%. But, in an alternative embodiment, guardslip-rings 9-1-2 and 9-1-3 (FIG. 26) or integrated lip guards 9-2-2(FIG. 27) are included to prevent inversion of any Belleville spring inthe stack, i.e., the frustum of the spring changing from pointing up todown or down to up. In one embodiment, a guard slip-ring isapproximately ¼ to approximately ½ the height of a single Bellevillespring. In another embodiment, the lip guards are approximately ±14 theheight of a Belleville spring. With lip guards, the guard slip-rings arenot necessary. The advantage of the guard slip-rings is that standardBelleville springs can be used in the spring stack. The advantage of thelip guards is that a separate part is not needed to eliminate springinversion.

A person with skill in the art would recognize that an inversion of anyspring can damage the spring and can change, at least minimally, thespring constant of the stack by converting a series configuration, inwhich the inverted spring is a part, into a parallel configuration.Unless the spring somehow re-inverts, this can have a deleterious effecton the intended operation of the device and should be avoided.Similarly, a parallel configuration would convert to a seriesconfiguration if only one spring inverted. Blocking rings or lip guardscan, thus, restrict the amount of linear displacement along the axialaxis since the springs are prevented from closing down completely.

There are several differences between Evan's instruction and that taughthere. First, the guard rings lie in the active displacement space of thespring(s) and do not require a ring stub between stacked springs. Thisreduces the height for the spring stack, a critical aspect since thespace height available is limited. The method taught here also providesthat the Belleville springs themselves can be modified in theirmanufacture with lip guards that perform the same function as the guardslip-rings, without requiring a separate device. The use ofdeflection-limiting guards avoids the inversion singularity that occursfor Belleville springs if 100% deflections are allowed. If seriescoupled springs are not matched in pairs, there is some risk of springinversion, in both Evan's scheme and the one instructed here. Forinstance, if one spring in series with another has a much smaller springconstant than the second spring in the pair, the softer spring mightinvert before the stiffer spring compress any significant amount. Toavoid this possibility, approximately-matched series coupled Bellevillesprings can be desirable.

For 10 springs in series, the total displacement equals 10·h_(e)millimeters, where h_(e), in millimeters, is the effective height of theBelleville spring, i.e., the actual amount the guards will allow eachspring to compress. For example, if h_(e) equals 0.224 mm, the springstack of 10 springs in series will compress a maximum of 2.24 mm.Therefore, a Belleville spring with height 0.32 mm and guard lips of0.08 mm will constrain a spring in a serial matched pair to compress nomore than 70% of its height. The effective height then is 0.7 times 0.32mm or 0.224 mm. At maximum compression, the central hydraulic cylinderand spring stack essentially becomes a fixed column between the FSUvertebrae that transmits any further compression forces to the FSUbelow.

The axial cylindrical joint, also called the central hydraulic cylinder,as noted in previous embodiments, comprises the combined elements of theinferior hydraulic cylinder 3-1, 4-1, and 5-1, and the superiorhydraulic cylinder, consisting of elements 6-1, 7-1, and 10-1 (FIGS.18-21). Thus, in one disclosed embodiment the slide block bearing 10-1-2fixedly joins to the top center of plate 10-1 with hydraulic portals10-1-3 drilled through the top surface to allow the transfer of fluidinto and out of the upper chamber. The segmented-wall inner core 7-1 iscentered and fixed to the underside of 10-1, such that the top surfacesof wall segments 7-1-2 alternate with slots 10-1-4. In an alternativeembodiment, the guard rings 4-1-2 and 6-1-2 are cut or molded as onepiece with the walls 4-1 and 6-1, this allows wall 4-1, whose outersurface 4-1-1 conforms to 6-1-1 and has hydraulic portals 4-1-3 drilledthrough the surface, to slide inside 6-1 from the top so the guard rings4-1-2 and 6-1-2 do not interfere with the assembly. Further, the outerwall 6-1 with hydraulic portals 6-1-3 drilled into its side is fixed tothe underside of 10-1, completing the superior hydraulic cylinder withinferior hydraulic cylinder wall 4-1 attached. In a further alternativeembodiment, the guard ring 6-1-2 is a separate item that can be weldedto the bottom of 6-1, then the superior and inferior hydraulic cylinderscan be constructed separately. The axial cylindrical joint can then beassembled by telescoping the inferior hydraulic cylinder into thesuperior hydraulic cylinder and welding guard ring 6-1-2 to the bottomof 6-1. In a specific embodiment, one can insert any of a variety ofresilient materials 15-1 and the spring stack 9-1 into the inferiorcylindrical chamber, where 5-1 and 7-1 project through the insidediameters of the springs and resilient material and 4-1 and 6-1 surroundand contain the springs and resilient material. A cutaway perspectiveview of the resulting axial cylindrical joint can be seen in FIGS. 9-11.

In one embodiment of the subject invention, the guard rings serve asjoint stops for the axial prismatic motion of the axial cylindricaljoint and prevent the device from separating when nominal forces attemptto hyperextend the FSU. In one embodiment, the guard rings circle theentire wall. In an alternative embodiment, the walls 4-1 and 6-1 can besegmented (three or four segments in a preferred embodiment) so thatparts of the walls have no guard rings and other parts do. In this way,the walls can be thicker at those points where there are no guard rings.For example, 4-1 could be uniformly thicker around the circumferencethan 6-1, except for those segments of the two walls which have guardrings (FIG. 35). Alternatively, the thickness of both walls could beequal, but larger than the segments with no guard rings. In a furtheralternative, each wall segment can have a different thickness, as longas the total additional thickness equals the gap width produced by theguard ring projections. The point of these alternatives is to addmechanical strength to the core with thicker walls so that larger sheerforces can be tolerated.

In one embodiment, the inferior hydraulic cylinder is constructed suchthat the slide block bearing 3-1-2 fixedly joins to the bottom center ofplate 3-1 with hydraulic portals 3-1-3 drilled through the top surfaceto allow the transfer of fluid into and out of the lower chamber. In afurther embodiment, the segmented-wall inner core 5-1 is centered andfixed to the topside of 3-1 such that bottom surfaces of the wallsegments 5-1-2 alternate with slots 3-1-4. Further, the bottom surfaceof wall 4-1 is centered and welded or otherwise fixed to the uppersurface of 3-1, completing the construction of the inferior hydrauliccylinder.

It should be understood that in an embodiment where the guard rings arepart of the wall structure, wall 4-1 is already telescoped into thesuperior hydraulic cylinder at this point in the construction. To finishthe axial cylindrical joint, attach inner core 5-1 to the top of uppersurface 3-1, place the spring stack 9-1, topped by resilient material15-1, onto the spring platform base 3-1, and then affix wall 4-1 to thetop of upper surface 3-1. The axial cylindrical joint can now slidealong and rotate about the axial axis 18-1 with mechanicallyprogrammable joint stops for each of the two degrees of freedom. Thespring stack and a resilient element provide resistance to compressionand maintain variable intervertebral spacing throughout motion in theFSU workspace. In particular, the spring stack parameters are designedso that the invention maintains normal disc spacing when the FSU is inthe normal position, but decreases the spacing during flexion andincreases the spacing during extension in order to mimic natural discbehavior.

With the axial cylindrical joint realized, the superior end can bejoined to the lateral rotation cylinder 11-1 and the inferior end can bejoined to the sagittal rotational cylinder 2-1, as described earlier.The novelty and importance of incorporating spring elements into amoving, central hydraulic cylinder acting as a cylindrical joint can nowbe explained. In this embodiment of the subject invention, the springelements move with the central hydraulic cylinder, a force acting on theFSU can sagittally and/or laterally rotate and/or slide the vertebralplates in the FSU moving frame and/or compress and rotate the centralhydraulic cylinder along/about its axis. This action can continue untiljoint limit stops are encountered or the force or moment-of-force alonga particular joint axis becomes zero. Any axial force component can tendto compress the spring elements along the preferred axis of the springelement and can be balanced out. Other components of the force can tendto activate the non-axial joint motions. At joint limit stops, therigidity of the device is capable of opposing any non-axial forces ormoment-of-force in the particular direction governed by thatjoint-at-limit. This feature of directing only normal forces onto thespring elements can be important for Belleville springs and other typesof axial springs, as they do not function well under non-normal forcesand is an important, novel element of this invention.

With the entire invention assembled the functions of cavity surfaces1-1-7, 1-1-8, and (FIG. 28) of the inferior vertebral plate 1-1 and12-1-7, 12-1-8, and 12-1-9 (FIG. 16) of the superior vertebral plate12-1 can be inferred, especially after careful consideration of FIGS. 9,10 and 11.

In one embodiment, the curvate, convex edge surface 3-1-1 conforms toconcave surface 1-1-8. In a particular embodiment, surface 1-1-8 iscylindrical with center of curvature on sagittal axis 16-1 and edgesurface 3-1-1 is spherical with center on sagittal axis 16-1. As theplate 3-1 rotates about 16-1, the surfaces 3-1-1 and 1-1-8 do notinterfere. When 3-1 sagittally translates to its extreme values, thesurface concavity at each end of the cylinder socket of 1-1 is not acontinuation of cylindrical surface 1-1-8, but is actually sphericalwith center coinciding with the moved center of surface 3-1-1. Thedisplaced center of 3-1-1 is still on 16-1, since the motion is alongthe direction of 16-1. The surfaces 3-1-1 and those at the end of thecylinder socket conform to one another and do not interfere duringsagittal rotation. This approach allows the walls of 1-1 to be thickerand more robust at the end of the socket concavity as opposed to arectangular shaped cut for the socket.

Surface 1-1-9 can conform to the lower surface of plate 3-1 and serve asa joint stop for sagittal rotation. The acute angle plane 1-1-9 makeswith the axial plane when the device is in normal position (FIG. 10)defines the maximum sagittal angle of rotation in one direction that theelements above the inferior vertebral device can rotate, in flexion(FIG. 9) or extension (FIG. 11). The planar cut 1-1-7 can prevent thewalls 6-1 (and necessarily walls 4-1) of the superior hydraulic cylinderfrom interfering with the inferior vertebral plate 1-1, even at maximumflexion or extension. In one embodiment, at the socket ends of 1-1, thesurface 1-1-7 becomes a rotated planar cut about the axial axis toprevent interference between 1-1 and 6-1 when the central hydrauliccylinder, at either extreme of sagittal translation, rotates about 16-1,even at maximum flexion and maximum extension.

In a further embodiment, the curvate, convex edge surface 10-1-1conforms to concave surface 12-1-8. In a particular embodiment, surface12-1-8 is cylindrical with center of curvature on lateral axis 17-1 andedge surface 10-1-1 is spherical with center on lateral axis 17-1. Inthis embodiment as the plate 10-1 rotates about 17-1, the surfaces10-1-1 and 12-1-8 do not interfere. When 10-1 laterally translates toits extreme values, the surface concavity at each end of the cylindersocket of 12-1 is not a continuation of cylindrical surface 12-1-8, butcan actually be spherical with center coinciding with the moved centerof surface 10-1-1. The displaced center of 10-1-1 can still be on 17-1,since the motion is along the direction of 17-1. The surfaces 10-1-1 andthose at the end of the cylinder socket are also able to conform to oneanother and do not interfere during lateral rotation. This approachallows the walls of 12-1 to be thicker and more robust at the end of thesocket concavity as opposed to a rectangular shaped cut for the socket.

Further, surface 12-1-9 can conform to the upper surface of plate 10-1and serves as a joint stop for lateral rotation. The acute angle thatplane 12-1-9 makes with the frontal plane, with the device in normalposition, defines the maximum lateral angle, in one direction, that thesuperior vertebral plate can rotate about 17-1. The planar cut 12-1-7can also prevent the walls 6-1 (and necessarily walls 4-1) of thesuperior hydraulic cylinder from interfering with the inferior vertebralplate 12-1, even at maximum flexion or extension. In a furtherembodiment, at the socket ends of 12-1, the surface 12-1-7 becomes arotated planar cut about the axial axis to prevent interference between12-1 and 6-1 when the central hydraulic cylinder, at either extreme oflateral translation, rotates about 17-1, even at maximum left or rightlateral bending.

A second embodiment (shown, for example, in FIGS. 29 and 30) is, ingeneral, the embodiment(s) described above, but with all lower orderpair joints replaced by higher order pairs. In a specific embodiment,ball or rod bearings are utilized running on raceway rods. The bearingsand rods in the subject invention can comprise any of a variety ofmaterials, including, for example, titanium-carbide-covered hardenedstainless steel, ultra-high-molecular-weight polyethylene or similarthermoplastic material; metal alloys, biocompatible materials and othersuitable materials or combinations thereof. In a specific embodiment,the raceway rods comprise hardened stainless steel. The general elementsn-2 of the second embodiment corresponds to the general elements n-1 ofthe first embodiment in overall shape and function, although theirdetail structures may differ substantially. In one embodiment, thesubject invention is basically cylindrical in shape with cylindricalsurfaces 12-2-12 and 1-2-12. However, a person with skill in the art andbenefit of the subject disclosure would be able to devise alternativeembodiments having different enclosing surfaces for the interiormechanism. Such alternative are within the scope of the subjectinvention.

In an alternative embodiment, lock keys are not needed to retain thesagittal 2-2 and lateral cylinders 11-2 into the inferior 1-2 andsuperior vertebral plates 12-2. Rather, to insure unbroken kinematiclinkage between the two vertebral plates, bearing elements canrotationally or slidingly lock the various joints together, namely, thelateral cylindrical joint (lateral revolute and slider joints) and thesagittal cylindrical joint (sagittal revolute and slider joints). In afurther embodiment, the lateral revolute joint consists of principalelements 11-2 and 12-2 (FIG. 29). The sagittal revolute joint consistsof principal elements of 1-2 and 2-2 (FIG. 47). The lateral slider jointconsists of principal elements 10-2 and 11-2 (FIG. 36). The sagittalslider joint elements consist of principal elements 3-2 and 2-2 (FIG.37).

FIG. 30 is an exploded perspective view of a second embodiment thatillustrates examples of the principal elements: 1) the inferiorvertebral plate with bearings 1-2, 2) the sagittal rotation cylinderwith bearings 2-2, 3) the spring base with sagittal slider raceway 3-2and bearing stop 3-2-5, 4) the spring element 9-1 (options andvariations in the spring elements are unchanged from the firstembodiment), 5) the inferior hydraulic cylinder outer walls 4-2 andinner core 5-2, 6) the superior hydraulic cylinder outer walls 6-2 andinner core 7-2, 7) the top plate 10-2 of the hydraulic cylinder withlateral prismatic raceway and bearing stops 10-2-5, 8) the lateralrotation cylinder with bearings 11-2, and 9) the superior vertebralplate with bearings 12-2.

FIG. 29, which is a quadrant cutaway perspective of an embodiment of theentire unit without the boot, shows a few more details. The distances20-1 and 20-2 show examples of the lateral and sagittal slider jointdisplacements. In one embodiment, distances 20-1 and 20-2 showapproximately one-half the lateral and sagittal slider jointdisplacements The segmented walls 5-2-2 and 7-2-2 and cores 5-2-1 and7-2-1 work as described above, except the walls do not penetrate intoplates 10-2 and 3-2. The guard rings 4-2-2 and 6-2-2 keep thetelescoping walls 4-2 and 6-2 from separating, and can, thus, serve as ajoint stop for the axial slider joint by limiting maximum extension.During sagittal rotations, plate 3-2 and elements attached above canrotate with cylinder 2-2 within the socket of 1-2.

Ball bearings 19-1 are shown in all the joints of FIG. 29, but, in analternative embodiment, they can be replaced by rod bearings,cylindrical bearings, or any of a variety of other suitable optionsknown to those skilled in the art. FIG. 29 does not show all the lateralrevolute joint bearing rods that can be utilized with embodiments of thesubject invention. But, the easily visible ones are 11-2-22, 11-2-24,11-2-25, 12-2-13, and 12-2-14. In one embodiment, each bearing raceway,in general, will have four bearing rods to support the joint loadsthrough ball bearings or other such elements. In a further embodiment,the bearing rods comprise hardened stainless steel. Embodimentsemploying bearing elements for the sagittal revolute joint and sagittalslider joint will be discussed in detail below. In one embodiment, thelateral revolute and slider joints function the same way as the sagittalrevolute and slider joints and have similar or identical structure asdescribed previously herein.

In the second embodiment, the inferior vertebral plate 1-2 can form fourbearing raceways 21-2-1, 21-2-2, 21-2-3, and 21-2-4 (FIG. 50) with thesagittal revolute cylinder with bearings, 2-2. Each of these racewayscan further have four rods each, comprising, for example, hardenedstainless steel: rods 1-2-21, 1-2-22, 2-2-24, and 2-2-25 for raceways21-2-1 and 21-2-4; and rods 1-2-17, 1-2-18, 2-2-22, and 2-2-23 forraceways 21-2-2 and 21-2-3 (FIG. 40). A perspective view of all thebearing elements for these four raceways is illustrated in FIG. 39.

An embodiment of a set of raceway rods for 21-2-1 and 21-2-4 is shown inFIG. 41. The exploded perspective view FIG. 42 shows the curvate bearingseparators 1-2-20 and bearing stops 1-2-19 that can be utilized with theinferior vertebral plate side of the raceway while the sagittal rotationcylinder portion has curvate bearing stops 2-2-27. The bearingseparators and stops of rods 1-2-21 and 1-2-22 are designed so as to beable to conform- to the cylindrical curvature of the rods and raceway.In one embodiment, with the ball bearings 19-1 and bearing separators1-2-20 in place, there is enough tolerance to slide the assembly intothe left side of the raceway 2-22-4 and the right side of raceway2-22-1, even with the sagittal cylinder in place within the inferiorvertebral plate socket. The bearing separators and stops can block onlyhalf of the raceway, whereby that, plus a small tolerance in the socketsize for 2-2, allows the rods 2-2-22 and 2-2-23 to slip into theright-side of 2-22-4 and the left side of 2-22-1. Bearing stops can bewelded or otherwise attached to the ends of the bearing rods. Thelengths of the configuration 2-2-22, 2-2-23, and stops 2-2-26, orequivalently, assuming the same length, of 2-2-24, 2-2-25, and 2-2-27,can determine the degrees of rotation of the sagittal cylinder withinthe inferior vertebral plate socket as the stops 2-2-26 and 2-2-27cannot get past the first ball bearing. When assembled with stops thebearings can lock the sagittal cylinder and the inferior vertebra platetogether and can simultaneously lock the bearings into place. A similartechnique can be applied to assembling bearings 19-1, bearing separators1-2-16, bearing stops 1-2-15 and 1-2-26, and curvate bearing rods1-2-17, 1-2-18, 2-2-22, and 2-2-23 into raceways 2-22-2 and 2-22-3(refer to FIGS. 43, 44, and 45, the latter highlighting the assembly ofjoint stops onto raceway rods).

In an alternative embodiment, bearings other than ball bearings can beused. For example, a rod-bearing 19-2 alternative to ball bearings isillustrated for one of the bearing assemblies in FIG. 46.

A set of bearing elements and four raceways identical to those of thesagittal rotation cylinder and inferior vertebral plate interface can beemployed for the lateral rotation cylinder 11-2 and superior vertebralplate 12-2 interface that allows bearing placement in like manner (seeFIG. 48). The cylindrical surface 12-2-10 and spherical surfaces 12-2-11can interface to conforming surfaces on rotation cylinder 11-2. Thesurfaces 12-2-7, 12-2-8, 12-2-9, and boot ring groove 12-2-6 can havethe same surface requirements and functions as the corresponding onesdescribed previously; likewise for surfaces 12-2-1, 12-2-2, 12-2-3,12-2-4. In one embodiment, planar surface 12-2-5 is cut back away fromthe end of the cylinder to allow enough lateral tolerance for assemblingthe locking bearing elements. In a further embodiment, the lateralrevolute and slider joints are identical in structure to the sagittalrevolute and slider joints, described above.

One embodiment of the placement of the sagittal rotation cylinder withbearings 2-2 into the inferior vertebral plate with bearings 1-2 isshown, for example, in the perspective view of FIG. 47, wherein aquarter section of the inferior vertebral plate with bearings 1-2 isremoved. Socket features visible here include 1-2-7, 1-2-8, and 1-2-9which can have the same function and curvatures as those surfacedescribed above, in the previous embodiment. Other socket surfacedetails appear in FIG. 49. Cylindrical concave surface 1-2-10 andspherical concave surfaces 1-2-11 can conform to corresponding surfaceson the sagittal rotation cylinder with bearings 2-2. In a furtherembodiment, planar surfaces 1-2-23, and 1-2-25 are bearing walls andcurvate surfaces 1-2-14 and 1-2-24 hold bearing raceway rods. In afurther embodiment, the planar surfaces 1-2-5 conform to the ends of thecylinder, but do not touch same in order to allow enough sagittal axisdisplacement for bearing insertion. Planar surfaces 1-2-13 do not comeinto contact with the cylinder 2-2, which can be completely supported bythe bearing elements of the revolute joint between 1-2 and 2-2.

FIG. 31 illustrates an embodiment of the sagittal rotation cylinder withbearings 2-2 in perspective from underneath, where the rotationalbearing elements have been removed from the raceways. FIG. 32illustrates an embodiment of cylinder 2-2 in perspective from above withthe sagittal slider bearings added (FIG. 33). FIG. 34 illustrates 2-2 inperspective from above without the sagittal slider bearing elements.

Structure, construction and function of the central hydraulic cylinderwith bearings of the second embodiment closely resemble the centralhydraulic cylinder of the first embodiment (FIGS. 35, 36, and 37). Forexample, both include an axial revolute joint and an axial slider joint.Slider joint construction differs considerably between the first andsecond embodiments and is discussed throughout the subject application.The embodiments disclosed herein disclose that component elements of thecentral hydraulic cylinder include centering and fixing 6-2 and 7-2 tothe under side of 10-2; centering and fixing 4-2 and 5-2 to the top sideof 3-2; and joining and slidably locking the two subassemblies togetherwith guard rings 6-2-2 and 4-2-2. Hydraulic portals 3-2-3, 4-2-3, 6-2-3,and 10-2-3 can be drilled through the associated elements of the centralhydraulic cylinder. These portals allow the passage of lubricating fluidand provide damping action to sudden compressive or extensive forcesacting on the cylinder.

Surfaces 6-2-1 and 4-2-1 can also be shaped into conforming walls withpolygonal cross sections or cross sections with combinations of curvatesegments. For all possible motions of the central hydraulic “cylinder”,convex surfaces 1-2-7 and 12-2-7, cut from the socket of the vertebralplates, can be constructed to conform to the rotated and translatedsurfaces of the central hydraulic “cylinder” without interference with1-2 or 12-2.

In further embodiments, the convex edge curvate surface 3-2-1 (10-2-1)and the concave wall surfaces 1-2-8 and 1-2-9 (12-2-8 and 12-2-9) of theinferior (superior) vertebral socket which plate 3-2 (10-2) encountersduring all its possible motions with respect to 1-2 (10-2) are allconformal to 3-2-1 (10-2-1) and do not interfere with each other, justlike the corresponding surface in the first embodiment do not interfere.Planar surface 1-2-9 (12-2-9) can act as a joint limit stop for thesagittal (lateral) revolute joints. Plate 3-2 (10-2) when rotated about16-1 (17-1) against surface 1-2-9 (12-2-9), will inhibit any furtherrotation of the sagittal (lateral) revolute joint.

A partial cutaway perspective of an embodiment of the sagittal revoluteand slider joints is shown in FIG. 38. The slider displacements 20-2 canbe seen in this view. Description of these joint embodiments and theirconstituent elements follow.

Many of the features of the sagittal rotation cylinder 2-1 (and byequivalence the lateral rotation cylinder 11-1) of the first embodimentare reflected in the second embodiment 2-2 (11-2). For example, in thesecond or alternative embodiment, surface 2-2-1 is cylindrical andsurfaces 2-2-2 are spherical with center of curvature on the sagittalaxis 16-1 and the planar surfaces 2-2-4 terminate the cylinder.

Surfaces in the second embodiment that can differ from the firstembodiment are now described. Furrows 2-2-5 allow two of the fourbearing rods welded, or otherwise fixedly attached, to plate 3-10, topass without touching 2-2. The equivalent embodiments of lateral sliderjoint rods 10-2-5 are seen in the lateral slider joint raceway 10-2-2 ofFIG. 36. FIG. 36 also illustrates an assembly of the lateral rotationcylinder with bearings 11-2 with the plate 10-2. After sliding thecylinder 11-2 into the raceway 10-2-2, the bearing-stop and joint-limitelements 10-2-4 can be press-fit, or otherwise fixed or positioned, intothe ends of raceway 10-2-2. The length of 10-2-4 can determine theamount of travel permitted by 11-2 within the slider joint. FIG. 37illustrates the same concept for the sagittal cylinder with thebearing-stop and joint-limit elements 3-2-4.

In a particular embodiment, cylindrical surface 2-2-6 does not touch1-2-13 (FIG. 49), letting all the bearing elements bear the loads.Concave surface 2-2-8, 2-2-9, 2-2-12, and 2-2-13 can serve as bearingrod revolute raceways, as discussed above. The sides 2-2-7, and 2-2-11of the revolute raceways can be cut planar. In a specific embodiment,the bearing elements of the subject invention can comprise hardenedstainless steel.

In a further embodiment, on top of the sagittal revolute cylinder withbearings is a bearing support structure 2-2-3 which accommodates thesagittal slider bearing elements (FIG. 32): ball bearings 19-1, bearingseparators 2-2-20, and bearing stops 2-2-10. The separators and stopscan be welded or otherwise fixedly attached to block 2-2-3. Concavesurface 2-2-28 and convex surface 2-2-29 of the bearing stops andseparators can also conform to the lateral surfaces of block 2-2-3 andfacilitate joining them to the block (FIG. 33). In a specificembodiment, projections 2-2-18 and 2-2-19 on the upper edge can behardened steel raceway rods. In an alternative embodiment, the entireblock 2-2-3 can be hardened stainless steel. These projections or rodscan provide interlock surfaces with the sagittal bearing elements whenthe sagittal cylinder is joined to the spring platform plate 3-1. Thediscussion here is meant in no way to limit or restrict other means ofimplementing slider bearings, or any of the bearing elements in thisinvention. A person with skill in the art and having benefit of thesubject application would be able to devise alternative bearing elementspresently or prospectively known that can be utilized with the subjectinvention. Such variations are considered to be within the scope of thesubject invention, unless specifically taught otherwise.

In a further embodiment, planar surfaces 2-2-14, 2-2-15, 2-2-16, and2-2-17 do not touch the lower planar surface of plate 3-1 in order totransfer all loads to the bearing elements as the sagittal slider jointbetween 3-2 and 2-2 operates. The planar surfaces 2-2-21 at either endof 2-2-3 can abut the ends of joint limit stops 3-2-4 at the two extremeslider positions. A similar description applies to correspondingelements of the lateral rotation cylinder with bearings.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

It should be understood that any reference in this specification to “oneembodiment,” “an embodiment,” “example embodiment,” “furtherembodiment,” “alternative embodiment,” etc., is for literaryconvenience. The implication is that any particular feature, structure,or characteristic described in connection with such an embodiment isincluded in at least one embodiment of the invention. The appearance ofsuch phrases in various places in the specification does not necessarilyrefer to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anyembodiment, it is submitted that it is within the purview of one skilledin the art to affect such feature, structure, or characteristic inconnection with other ones of the embodiments.

The invention has been described herein in considerable detail, in orderto comply with the Patent Statutes and to provide those skilled in theart with information needed to apply the novel principles, and toconstruct and use such specialized components as are required. However,it is to be understood that the invention can be carried out byspecifically different equipment and devices, and that variousmodifications, both as to equipment details and operating procedures canbe effected without departing from the scope of the invention itself.Further, it should be understood that, although the present inventionhas been described with reference to specific details of certainembodiments thereof, it is not intended that such details should beregarded as limitations upon the scope of the invention except as and tothe extent that they are included in the accompanying claims.

1. A prosthetic device for approximating spinal disc movementcomprising: a spring-dashpot system having a superior end and aninferior end that includes, a superior segmented-wall inner core; aninferior segmented-wall inner core that engages with the superiorsegmented-wall inner core; one or more spring elements positioned aroundthe segmented-wall inner cores, an inferior hydraulic cylinder wallpositioned around the one or more spring elements, and a superiorhydraulic cylinder wall that slidably engages telescopically with and isinseparable from the inferior hydraulic cylinder wall, such that theinferior and superior hydraulic cylinder walls confine the one or morespring elements; a superior hydraulic cylinder plate having a top andbottom side, wherein the bottom side is fixedly attached to the superiorend of the spring-dashpot system; a first slide bearing block fixedlyattached to the top side of the superior hydraulic cylinder plate; afirst rotation cylinder with a bearing raceway slidably affixed to thefirst slide bearing block; an inferior hydraulic cylinder base having atop and bottom side, wherein the top side is fixedly attached to theinferior end of the spring-dashpot system; a second slide bearing blockfixedly attached to the bottom side of the inferior hydraulic cylinderbase; a second rotation cylinder with a bearing raceway slidably affixedto the second slide bearing block; at least one means for attachment toa first vertebra being operably connected to the first rotationcylinder; and at least one means for attachment to a second vertebrabeing operably connected to the second rotation cylinder, such that whensaid device is implanted in the spine with said attachment means engagedwith a first and second vertebra, said device forms a kinematic chain ofinseparably connected, articulating components between said first andsecond vertebra, wherein said device can provide at least one and up tothree independent rotational degrees of freedom and at least one and upto three independent linear degrees of freedom.
 2. The device, accordingto claim 1, further comprising at least one hydraulic portal.
 3. Thedevice, according to claim 1, further comprising at least one indentedwall slot within the bottom side of the superior hydraulic cylinderplate and capable of cooperatively engaging with the inferiorsegmented-wall inner core.
 4. The device, according to claim 3, furthercomprising at least one indented wall slot within the top side of theinferior hydraulic cylinder base and capable of cooperatively engagingwith the superior segmented-wall inner core.
 5. The device, according toclaim 1, further comprising a toroidal belt at least partiallysurrounding the spring-dashpot system.
 6. The device, according to claim3, wherein the toroidal belt comprises a solid fiber-weave-embeddedelastomer material.
 7. The device, according to claim 6, wherein thetoroidal belt further comprises an interior cushioning material.
 8. Thedevice, according to claim 7, wherein the cushioning material is air,compressible fluid, or hydrogel material.
 9. The device, according toclaim 5 wherein the toroidal belt is unconnected and can move freelyaround the spring-dashpot system.
 10. The device, according to claim 5,wherein the toroidal belt is fixedly engaged with each of said means forattachment to the vertebrae and surrounds the functional elements. 11.The device, according to claim 1, further comprising at least onebearing stop.
 12. The device, according to claim 1, further comprising alocking key slot within at least one of the rotation cylinders forengaging with a locking key.
 13. The device, according to claim 12,wherein the means for attachment to the vertebrae is affixed to at leastone of the rotation cylinders utilizing the locking key.
 14. The device,according to claim 1, wherein the one or more spring elements compriseone or more Belleville springs.
 15. The device, according to claim 14,further comprising at least one matched-pair, or approximatelymatched-pair of Belleville springs.
 16. The device, according to claim15, further comprising at least one guard ring between matchedBelleville spring pairs.
 17. The device, according to claim 15, furthercomprising a raised lip on at least one of the Belleville springs in anapproximately matched-pair.
 18. The device, according to claim 1,further comprising at least one lock ring on both the inferior hydrauliccylinder wall and the superior hydraulic cylinder wall, wherein the lockrings render the cylinder walls inseparable.
 19. The device, accordingto claim 1, further comprising at least one locking projection on atleast one of the slide bearing blocks that, when engaged with thebearing raceway, renders the slide bearing block and rotating cylinderinseparable.
 20. The device, according to claim 1, wherein at least oneof the components comprises titanium steel, titanium-carbide-coatedstainless steel, bio-inert hardened stainless steel, polyurethane,polyurethane thermoplastic, cobalt-chromium-molybdenum alloy, plastic,ceramics, glass, or other materials or combinations thereof.
 21. Thedevice, according to claim 1, further comprising an impermeable bootfixedly engaged with each of said means for attachment to the vertebraeand surrounding the functional elements.
 22. The device, according toclaim 21, further comprising a biocompatible lubricant sealed within theboot.
 23. The device, according to claim 21, wherein the boot comprisesa fiber-reinforced elastomer matrix.
 24. The device, according to claim23, wherein the fiber-reinforcement comprises a spherical cross weave,such that the weave direction of the embedded fibers is diagonalrelative to the central axis of the boot structure.
 25. The device,according to claim 23, wherein the elastomer matrix is a flexiblesilicon.
 26. The device, according to claim 5, further comprising animpermeable boot fixedly engaged with each of said means for attachmentto the vertebrae and to the toroidal belt.
 27. The device, according toclaim 26, further comprising a biocompatible lubricant sealed within theboot.
 28. The device, according to claim 26, wherein the boot comprisesa fiber-reinforced elastomer matrix.
 29. The device, according to claim28, wherein the fiber-reinforcement comprises a spherical cross weave,such that the weave direction of the embedded fibers is diagonalrelative to the central axis the boot structure.
 30. The device,according to claim 28, wherein the elastomer matrix is a flexiblesilicon.
 31. The device, according to claim 26, wherein the toroidalbelt comprises a solid fiber-weave-embedded elastomer material.
 32. Thedevice, according to claim 31, wherein the toroidal belt furthercomprises an interior cushioning material.
 33. The device, according toclaim 32, wherein the cushioning material is air, compressible fluid, orhydrogel material.
 34. A prosthetic device for approximating spinal discmovement comprising: a spring-dashpot system having a superior end andan inferior end that includes, a superior segmented-wall inner core; aninferior segmented-wall inner core that engages with the superiorsegmented-wall inner core; one or more spring elements positioned aroundthe segmented-wall inner cores, an inferior hydraulic cylinder wallpositioned around the one or more spring elements, and a superiorhydraulic cylinder wall that slidably engages telescopically with and isinseparable from the inferior hydraulic cylinder wall, such that theinferior and superior hydraulic cylinder walls confine the one or morespring elements; a superior hydraulic cylinder plate having a top andbottom side, wherein the bottom side is fixedly attached to the superiorend of the spring-dashpot system and wherein the top side comprises afirst slider joint raceway; a first rotation cylinder comprising alinear bearing structure that is slidably connected to the first sliderjoint raceway; an inferior hydraulic cylinder base having a top andbottom side, wherein the top side is fixedly attached to the inferiorend of the spring-dashpot system and the bottom side comprises a secondslider joint raceway; a second rotation cylinder comprising a linearbearing structure that is slidably connected to the second slider jointraceway; at least one means for attachment to a first vertebra beingoperably connected to the first rotation cylinder; and at least onemeans for attachment to a second vertebra being operably connected tothe second rotation cylinder, such that when said device is implanted inthe spine with said attachment means engaged with a first and secondvertebra, said device forms a kinematic chain of inseparably connected,articulating components between said first and second vertebra, whereinsaid device can provide at least one and up to three independentrotational degrees of freedom and at least one and up to threeindependent linear degrees of freedom.
 35. The device, according toclaim 34, further comprising at least one hydraulic portal.
 36. Thedevice, according to claim 35, further comprising one or more bearingspositioned within at least one of the slider joint raceways such that alinear bearing structure when slidably connected is supported by andmoves upon the one or more bearings.
 37. The device, according to claim36, further comprising at least one bearing stop and/or bearingseparator.
 38. The device, according to claim 36, wherein the one ormore bearings are ball bearings, rod bearings, textured rod bearings,curvate rod bearings, or combinations thereof.
 39. The device, accordingto claim 34, further comprising at least one curvate bearing racewaywithin at least one of the rotation cylinders.
 40. The device, accordingto claim 39, further comprising one or more bearings positioned withinthe at least one curvate bearing raceway.
 41. The device, according toclaim 40, wherein the means for attachment to a vertebra is operably andinseparably connected to a rotation cylinder utilizing the one or morebearings.
 42. The device, according to claim 41, wherein the one or morebearings are ball bearings, rod bearings, textured rod bearings orcombinations thereof.
 43. The device, according to claim 39, furthercomprising at least one bearing stop.
 44. The device, according to claim34, further comprising a toroidal belt at least partially surroundingthe spring-dashpot system.
 45. The device, according to claim 44,wherein the toroidal belt comprises a solid fiber-weave-embeddedelastomer material.
 46. The device, according to claim 45, wherein thetoroidal belt further comprises an interior cushioning material.
 47. Thedevice, according to claim 46, wherein the cushioning material is air,compressible fluid, or hydrogel material.
 48. The device, according toclaim 44, wherein the toroidal belt is unconnected and can move freelyaround the spring-dashpot system.
 49. The device, according to claim 44,wherein the toroidal belt is fixedly engaged with each of said means forattachment to the vertebrae and surrounds the functional elements. 50.The device, according to claim 34, wherein the one or more springelements comprise one or more Belleville springs.
 51. The device,according to claim 50, further comprising at least one matched-pair ofBelleville springs.
 52. The device, according to claim 51, furthercomprising at least one guard ring between matched Belleville springpairs.
 53. The device, according to claim 51, further comprising araised lip on at least one of the Belleville springs in a matched pair.54. The device, according to claim 34, further comprising at least onelock ring on both the inferior hydraulic cylinder wall and the superiorhydraulic cylinder wall, wherein the lock rings render the cylinderwalls inseparable.
 55. The device, according to claim 34, wherein atleast one of the components comprises titanium steel,titanium-carbide-coated stainless steel, bio-inert hardened stainlesssteel, polyurethane, polyurethane thermoplastic,cobalt-chromium-molybdenum alloy, plastic, ceramics, glass, or othermaterials or combinations thereof.
 56. The device, according to claim34, further comprising an impermable boot fixedly engaged with each ofsaid means for attachment to the vertebrae and surrounding thefunctional elements.
 57. The device, according to claim 56, furthercomprising a biocompatible lubricant sealed within the boot.
 58. Thedevice, according to claim 56, wherein the boot comprises afiber-reinforced elastomer matrix.
 59. The device, according to claim58, wherein the fiber-reinforcement comprises a spherical cross weave,such that the weave direction of the embedded fibers is diagonalrelative to the central axis of the boot structure.
 60. The device,according to claim 58, wherein the elastomer matrix is a flexiblesilicon.
 61. The device, according to claim 44, further comprising animpermeable boot fixedly engaged with each of said means for attachmentto the vertebrae and to the toroidal belt.
 62. The device, according toclaim 61, further comprising a biocompatible lubricant sealed within theboot.
 63. The device, according to claim 61, wherein the boot comprisesa fiber-reinforced elastomer matrix.
 64. The device, according to claim63, wherein the fiber-reinforcement comprises a spherical cross weave,such that the weave direction of the embedded fibers is diagonalrelative to the central axis the boot structure.
 65. The device,according to claim 63, wherein the elastomer matrix is a flexiblesilicon.
 66. The device, according to claim 26, wherein the toroidalbelt comprises a solid fiber-weave-embedded elastomer material.
 67. Thedevice, according to claim 66, wherein the toroidal belt furthercomprises an interior cushioning material.
 68. The device, according toclaim 67, wherein the cushioning material is air, compressible fluid, orhydrogel material.
 69. A method for approximating spinal disc movementutilizing a device comprising: a spring-dashpot system having a superiorend and an inferior end that includes, a superior segmented-wall innercore; an inferior segmented-wall inner core that engages with thesuperior segmented-wall inner core; one or more spring elementspositioned around the segmented-wall inner cores, an inferior hydrauliccylinder wall positioned around the one or more spring elements, and asuperior hydraulic cylinder wall that slidably engages telescopicallywith and is inseparable from the inferior hydraulic cylinder wall, suchthat the inferior and superior hydraulic cylinder walls confine the oneor more spring elements; a superior hydraulic cylinder plate having atop and bottom side, wherein the bottom side is fixedly attached to thesuperior end of the spring-dashpot system; a first slide bearing blockfixedly attached to the top side of the superior hydraulic cylinderplate; a first rotation cylinder with a bearing raceway slidably andinseparably affixed to the first slide bearing block; an inferiorhydraulic cylinder base having a top and bottom side, wherein the topside is fixedly attached to the inferior end of the spring-dashpotsystem; a second slide bearing block fixedly attached to the bottom sideof the inferior hydraulic cylinder base; a second rotation cylinder witha bearing raceway slidably and inseparably affixed to the second slidebearing block; at least one means for attachment to a first vertebrabeing operably connected to and inseparable from the first rotationcylinder; and at least one means for attachment to a second vertebrabeing operably connected to and inseparable from the second rotationcylinder, said method comprising securing the device within the spine ofan animal utilizing said means for attachment to a first and secondvertebra, such that said device forms a kinematic chain of inseparablyconnected, articulating components between said first and secondvertebra, wherein said device can provide at least one and up to threeindependent rotational degrees of freedom and at least one and up tothree independent linear degrees of freedom.
 70. A method forapproximating spinal disc movement utilizing a device comprising: aspring-dashpot system having a superior end and an inferior end thatincludes, a superior segmented-wall inner core; an inferiorsegmented-wall inner core that engages with the superior segmented-wallinner core; one or more spring elements positioned around thesegmented-wall inner cores, an inferior hydraulic cylinder wallpositioned around the one or more spring elements, and a superiorhydraulic cylinder wall that slidably engages telescopically with and isinseparable from the inferior hydraulic cylinder wall, such that theinferior and superior hydraulic cylinder walls confine the one or morespring elements; a superior hydraulic cylinder plate having a top andbottom side, wherein the bottom side is fixedly attached to the superiorend of the spring-dashpot system and wherein the top side comprises afirst slider joint raceway; a first rotation cylinder comprising alinear bearing structure that is slidably connected to the first sliderjoint raceway; an inferior hydraulic cylinder base having a top andbottom side, wherein the top side is fixedly attached to the inferiorend of the spring-dashpot system and the bottom side comprises a secondslider joint raceway; a second rotation cylinder comprising a linearbearing structure that is slidably connected to the second slider jointraceway; at least one means for attachment to a first vertebra beingoperably connected to the first rotation cylinder; and at least onemeans for attachment to a second vertebra being operably connected tothe second rotation cylinder, said method comprising securing the devicewithin the spine of an animal utilizing said means for attachment to afirst and second vertebra, such that said device forms a kinematic chainof inseparably connected, articulating components between said first andsecond vertebra, wherein said device can provide at least one and up tothree independent rotational degrees of freedom and at least one and upto three independent linear degrees of freedom.